Mobile video chat company Tango announced today that it has updated its iPhone OS app to support the fourth-generation iPod Touch, which features front and rear-facing cameras like the iPhone 4.

With this update, Tango is now a bigger threat to Apple’s own FaceTime video chat, which is built in to the iPhone 4 and fourth-generation iPod Touch. The big difference with Tango is that users can also video chat over 3G (FaceTime is restricted to WiFi only), and they can video chat with friends on Android phones. Tango’s interface is as simple and easy to use as FaceTime, and it’s also easy to find your friends on the service as the app automatically combs through your phone’s contacts.

We reported in October that Tango’s iPhone and Android apps were wildly popular, garnering 1 million downloads in a mere 10 days after they launched. Tango hit 3 million downloads by the end of November. The company tells me that its usage doubled over the Thanksgiving holiday.

Tango says that 70 percent of its members perform spontaneous video calls with their friends and family, and they do so more than once a week. I argued earlier this year that the iPhone 4 would spur on innovation in mobile video chat, and we’re clearly seeing that happen with Tango’s apps.

In a video chat interview, Tango’s executives mentioned that the service is using peer-to-peer (P2P) technology to handle the video calls. That means the service is highly scalable and doesn’t rely entirely on the speed of the company’s servers. Tango has six patents pending surrounding its video chat technology, three of which are related specifically to its P2P technology implementation.

A recent Android update allows Tango’s app to take advantage of Cloud to Device Messaging (C2DM) technology, a feature of the Android OS that lets push notifications get sent for Tango calls without having to run the app in the background — saving precious battery life in the process.

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.