SAN FRANCISCO — Yo is a super-simple app that does one thing. It lets you send a message that says “Yo.” And that’s it.

Yo is possibly the most polarizing app in history. There are no in-betweens with Yo: You either love it or hate it.

But love it or hate it, Yo is wildly popular. It was released on April 1, and added 1 million users during a whirlwind 4-day period in June.

And today it is hitting 2 million downloads, the Yo founders revealed.

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2 million downloads makes it easy to shrug off the “your app is stupid” comments at the app stores — though the founders certainly acknowledge that their app’s simplicity borders on stupidity.

“There’s a fine line between stupid and genius,” said Yo co-founder Moshe Hogeg onstage at the MobileBeat conference Tuesday. He spoke along with Yo co-founder and “CEYo” Or Arbel.

You can see the whole video of their entertaining 20-minute onstage chat below.

Yo Founders Moshe Hogeg and Or Arbel at MobileBeat

Hogeg and Arbel say that the reason people like Yo is not because it’s a novelty, but rather because users can define for themselves what a Yo message means.

“Sometimes the message is the message,” Hogeg said.

Hogeg explains that the meaning of a Yo is often dictated by the context in which it’s sent.

When you see a call coming in from a friend, much of the time you already know what it’s about, Hogeg said.

Or, it can be up to the user (and the users’ contacts) to define what a Yo means.

“Yo can mean anything because it means nothing,” said Arbel.

(If you haven’t noticed yet, the men from Yo are masters of the sound-bite.)

For Hogeg and his wife, a Yo!  means “I love you.”

For one developer working with IFTTT.com and the Yo API, a Yo can be used as a switch to turn on lights or the air conditioning in a house.

Yo’s aren’t always fun. One developer created an app that sends followers a Yo when a missile is on its way to a target in Israel, so the Yo serves as a warning that it’s time to take cover. (Yo is an Israeli company.)

The sobering fact is that Yo might be just another novelty that people will abandon by this time next year.

And Hogeg’s and Arbels’s answer to the “novelty” problem is an all-too-common one.

“If companies and brands find ways to use the app it will be a success,” Arbel said. “If it’s just something you use to annoy your friends at the office, well . . . ”

Whether or not some big brand is creative enough to use contextual messaging may be the difference between riches and obscurity for Yo.

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