Thirteen employees. No revenue. One billion dollars. Facebook's purchase in April of photo-sharing service Instagram may have smacked more of a winning Powerball ticket than a by-the-numbers business deal. Even so, Silicon Valley is hot on the trail of the next billion-dollar idea. Here are a few of the top contenders.

Evernote

Compared with Instagram, Evernote isn't nearly as sexy. Still, the note-taking app (and Inc.'s 2011 Company of the Year) has grown so popular that rumors were swirling in April that its fifth funding round may value the company at $1 billion. In 2011, Evernote doubled its user base, to 20 million.

Pinterest

The odds-on favorite to snatch the next megadeal, Pinterest, a scrapbook-meets-Facebook service, rocketed from about half a million monthly visits a year ago to 104 million in March. That's the third-largest traffic number, behind only Twitter and Facebook, according to Experian Hitwise.

Rovio

Like one of its Angry Birds, Finland-based Rovio has catapulted the mobile gaming industry to new heights. Downloads of the company's games are expected to top two billion by the end of 2012. Plus, Rovio keeps expanding into new markets, such as plush toys, pajamas, and an upcoming TV series.

Spotify

This U.K.-based music streaming service managed to hit a sweet spot of compromise with the music industry. In exchange for access to a catalog of hit songs, users agree to broadcast their music tastes to their Facebook friends. Spotify now boasts 10 million usersthree million of whom are paid subscribers.