You might find this answer surprising, especially from a former executive at LinkedIn (I'm currently the CEO at Wealthfront).  But the short answer is, yes, in many cases, there is a professional necessity for Facebook.

Let me first get my obvious bias out of the way.  As one of the people who had a hand in building out LinkedIn, I truly believe that there is a valuable and natural separation between your professional identity and your personal identity.  As a result, I would counsel any professional to take great care in how they present themselves on LinkedIn.

However, Facebook is ubiquitous, and it is an easily accessible and common search engine for people.  It is well indexed in search engines like Google.

Chances are, if someone is looking for information about you, as a professional, they will definitely see your LinkedIn profile.  There is also a strong chance they will end up looking for your Facebook profile as well.

Why?

A few reasons really:

• Completeness.  It's easily accessible and fairly common.  It feels like a reasonable part of due diligence.
• Curiosity. Facebook plays strongly to our curiosity to know more about people.  The fact that there is a strong personal / professional split can make people distinctly curious about your split.
• Psychographics.  Whether correct or incorrect, by looking at your social behavior on sites like Facebook, Instagram, Twitter or Pinterest, other professionals may believe they can get a better sense of who you are as a person. 
• Transparency.  Finding a Facebook profile reassures others that you have "nothing to hide."  In a world where Facebook is ubiquitous, not finding a profile or not finding sufficient information may inadvertently beg questions.
  

Facebook offers quite a few tools to control privacy and sharing.  A few recommendations to consider:

• Tightly Control Your Public Profile on Facebook.  To the extent you can, make sure your public profile on Facebook (including public shares and photos) are carefully curated to present the image you would be comfortable with a business partner or colleague seeing.  This doesn't mean hiding your humanity, but it likely means publishing only a limited amount that frames who you are.
• Don't Make Personal Squabbles Public.  Too many times, relatively well-behaved adults present a poor version of themselves when they get into arguments in the comments on Facebook posts, not realizing those comments appear on a public share of a friend.  My advice? From a career perspective, save arguments for private messages and other communication.  Assume that every comment you make on a Facebook share could be public.
  
• Watch for inconsistency.  When people view multiple social profiles, inconsistency (or worse, hypocrisy) tends to stand out.  Presenting yourself as serious and hard-working on LinkedIn, while portraying an active and wild night-life on Facebook will beg questions.

As a final note, Facebook is most useful professionally as an alternate messaging platform for close friends & colleagues.  We live in a world where many people split their attention across too many communication channels.

As a result, I now know people who only reliably respond to text messages, answering in minutes where an email will sit unanswered for days.  I know people who only respond to DMs on Twitter, or messages on WhatsApp.  I also know people who only respond reliably to messages on Facebook. 

So if your goal is fast & efficient communication, it's worth having a channel to your friends & colleagues who prefer Facebook (even if you don't.)

Necessity is a strong word.  You can obviously succeed professionally without Facebook.  However, I would argue that using Facebook effectively has distinct value as a professional and there are situations where it can be considered a necessity.

I've gotten many deals on Facebook and in my recent talks to executives around the world nearly ALL of them are on it. So if you don't want to reach the best audience and you don't want to do business with them, sure, don't be on it. For me, and for Rackspace, it's a competitive advantage. A really big one.

Absolutely yes.

Adam Nash has already covered very well why Facebook is important for your professional reputation. I agree that a person with no visible Facebook page will not be as credible as someone with a well rounded online footprint. Potential employers will often look at your public Facebook profile and even if they don't they will see it appear in a Google search for your name.

There is, however, another important reason for professionals to use Facebook. Very often the person you need to contact is not in your LinkedIn network but is known to someone in your personal network. By mentioning on Facebook that you are looking for something or someone maybe one of your friends will be able to help. Perhaps it is a distant relative you forgot you were connected to, or maybe it is an old school friend. Facebook connects you with people you often don't realize you were connected to.

Many times I have seen people trying to find a connection at a company. Often they find one through a LinkedIn search but when that doesn't work just mentioning it on Facebook can uncover some surprising connections. You never know who your friends know until you ask.

Necessity? Probably not, hardly anyone would ever ask in a professional setting if you have a facebook profile or not.

That being said it can be positive or negative for you professional profile. Many employers will try to find you on social networks to try and get a better idea of who you are, what you show publicly on facebook could be positive or negative to them.

So all in all, don't worry about deactivating your profile, you may or may not be asked about it, but that's about as far as it goes.

Thanks to Anonymous for the A2A.

Just wanna make sure I am answering the right question. :)

If your question can be rephrased as "Should I use Facebook at least sometimes for professional communication?" - my answer would be, yes, why not, if you get leads or contacts through Facebook, and that is the platform they prefer, you should certainly use it.

If, however, you are asking "Is it OK to use Facebook for most of my professional communication, the same way I use LinkedIn?" - I would say, no, you shouldn't. Given the nature of Facebook communication, the needs of the audience and the type of content shared and followed by most users, I can hardly see any value in doing it.

Hope this could be of some help.

Interesting answers! I have to say I have had quite a different experience than most of the people here. I use Facebook extensively for professional purposes.

I am constantly talking to people in Facebook groups, whether it's a group for entrepreneurs in Madrid, expats in Spain, communications professionals from my alma mater, etc. It's mainly been a way to network with people with similar interests as me, as well as connect with customers and potential customers.

LinkedIn has similar groups, and I use them in a similar way. However, people are really guarded about what they share on LinkedIn. Once you become someone's friend on Facebook you've really entered their circle of trust. And, for me, these friendships have lead to a lot of professional synergies that, at least for me, are more valuable than the "professional contacts" I've made via LinkedIn. But, again, that's just me. I've always been a Facebook fan.

Is Facebook a professional necessity? It depends on your profession. But it can certainly be a useful tool.

I have not found any use for Facebook, professionally.

I agree with many of the comments here that it is an important tool for professional purposes, used in several different ways noted above.   This is especially true if you're working in the field of consumer applications, as it's where many of your customers are and is also likely a primary channel to reach them.

No, I can't say it's a necessity. It is something which can fall into the category of "Nice to have or good to have". To support my argument, let me list down a few reasons:

• Family: there is a difference (and distance) between a professional and personal life, and it should be maintained that way. For example, Facebook may have details on my family/partner (read my sexual preference), kids, friends and so on. I don't  want to broadcast my partner and family details to my co-workers and professional network. Even in liberal countries like US  people don't accept (some don't like and some hate it) the same-sex relationship on various grounds; forget about the old fashioned countries like India and other parts of the world.

• Personal life: our workplace has people from various groups, religions, cultures and so on. People tend to follow (or forced to follow) a neutral approach (in office or professional environment) towards everyone despite the differences. But, in reality, there are thousands of cases when a colleague is looked upon with some nasty eyes (or comments).....my personal experience of working in a multi-national company across three continents further proves it. People are more vocal about their interest and beliefs on Facebook. Not to forget, some people don't even think about a sanitize language before making it loud in Facebook.

• Gender: this point is mainly for female colleagues who may not feel comfortable connecting with other male colleagues on Facebook. Women in all cultures respect a certain amount of privacy and after connecting on Facebook, you may provide some unwanted details to your colleagues or professional network (for example, your relationship status, your vacation photographs, etc.). Women in our world have a good amount of freedom only in a few countries, mainly developed countries in the west with some more countries. For rest of the world, women are still struggling to get a decent representation in all aspects of professional life.

Finally, in big size multinational organizations, the employee strength is more than 100,000, and if we start projecting Facebook as an extension of Linkedin and call it a part of professional network.....many of us would not be comfortable with this idea.

People are more likely to add an unknown colleague at Linkedin as compared to Facebook.

Facebook provides you a little bit more freedom when it comes to express yourself (to the external world), but, if you want to keep the same level of exposure to the world what you have it on Linkedin, it's a fair deal.

My view is a minority view, but I have derived benefits from connecting to several of my coworkers on Facebook. In my case, it enhances the work relationship.

It can be used by the recruiters to determine the attitude and perhaps authenticity too..

Other than that, I see none..

A2A
I do not work in the industry yet but I tell you from interactions I've had.
Many HRs from companies and actual employees and company advisers have themselves told me they use a person's social networking profile to determine his or her attitude since that is the place where they are the freest.
And a social media profile also suggests market trends. Xiaomi informed that it had chosen India as it's biggest market based on the popularity in Indian markets of that brand.

In my opinion I don't find necessity for Facebook even if you are not geek.

For me Facebook = Waste of time  !!

No there is no professional necessity for facebook.

Facebook is not meant for Professional Networking. You will end-up in more completions if you use it for professional contacts.
Most difficult part of FB is any friend (or anyone) can tag you in any event or photo's which may not be yours interest and could lead to misconception with your professional contacts.
So I will recommend don't use FB for professional n/w.