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tango screenshotTango, a video-calling app for the iPhone and devices running Google’s Android mobile operating system, has raised $8.7 million of a $10 million funding round, according to a filing with the Securities and Exchange Commission.

The Tango app lets tablet and smartphone users make video calls to other Tango users, much like Apple’s FaceTime app for the iPhone 4. The service works over 3G and 4G networks, whereas FaceTime only works over wi-fi.

The service relies on peer-to-peer networking that most file-sharing applications rely on today. That means the technology doesn’t rely entirely on servers and can scale up — as it already has, because the app turned out to be pretty popular. Smartphone and tablet users downloaded it around 1 million times in its first 10 days after release.

Based in Palo Alto, Calif., Tango has raised $5 million in funding from individual investors including Bill Hambrecht, Michael Birch, Bill Tai, and Daniel Scheinman. This brings its total funding up to $13.7 million.

Tango didn’t give extra details about the funding round — such as whether it’s an institutional round or a seed round. The funding appears to be convertible debt, but the company couldn’t confirm the nature of the financing over the phone right away. VentureBeat has contacted Tango and will update the story as soon as we get that information.