Yes, Yo still exists.

Yo, an app that just lets users send the word "yo" to each other, announced that it has topped 1 million users. Most of those users — about 950,000 — were added in just the four-day period after Yo first gained mainstream attention last week.

At the time of publication, the app was up to 1.13 million users, according to the latest data provided to Mashable.

The app launched quietly in April, but took off on Wednesday after The Financial Times published an article about Yo, revealing that it had somehow managed to attract $1 million in funding. That set off a barrage of articles and social media posts questioning how an app that only lets you "yo" people could raise that much money.

We welcome our 1,000,007 user! After 4 days, we are humble and thankful for your love! pic.twitter.com/OwbjeQ2Sok

— Yo (@YoAppStatus) June 22, 2014

Moshe Hogeg, one of the app's cofounders, revealed more recently that investors were willing to invest more than $2.5 million in the app, but the founders opted for a lower amount.

All that attention helped Yo attract hundreds of thousands of users and crack the Top 5 free apps in Apple's App Store. The app has since dipped a bit in the rankings, though it remains in the Top 10.

Some assumed on Friday that Yo's days might be numbered after its creators revealed that the app had been hacked, potentially exposing users' phone numbers. Yo ended up working with the hackers to resolve the issue.

"Yo started as a weekend project and exploded a little too soon," Or Arbel, the app's creator, wrote about the hack in a blog post. "We were just finishing up re-writing the infrastructure in a proper and secure way, as suitable for production grade apps, when it suddenly blew up and went viral."

On Monday, Yo announced that it was making its API available to developers. The goal of the creators is to turn Yo into a notification tool for publishers, brands and individuals.

"If you think this is just an app that says 'yo,' you are getting it wrong," Arbel told Mashable in an earlier interview. "It's a new way to get lightweight, non-intrusive notifications. We are here to cut through the noise. We like to call it context-based messaging."

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