In December I wrote about Thwapr, a video-sharing service for mobile videos. Instead of building apps for smartphones, Thwapr chose to make its videos auto-playable on as many phones — smart or dumb — as possible without any software downloads, including apps.

This week, Thwapr has added one-click Facebook and Twitter sharing to video clips hosted at Thwapr, as well as a “Thwap It” button to share with one or more mobile phones.

The catch is: There’s no catch. You needn’t sign up with Thwapr to share or receive a clip. So it’s just like YouTube. Thwapr makes its money by serving ads and by selling premium placement to brand managers.

CTO Eric Hoffert gave me a demo over the phone. As you can see on this demo clip, all you need to do is click the Facebook or Twitter button to send a tagged link and comment to your own network. Tweets get a #thwapr hashtag.

The Thwapit button lets you send a message to someone else’s mobile phone, with a link to a video that will automatically play on more than 200 models of phone.

The most serendipitous part of the service, Hoffert says, is that many recipients aren’t aware their phone can play video clips until they click. Says Hoffert: “On a BlackBerry 8330 or a Droid on Verizon, Thwapr serves streaming video in RTSP format. On a more modestly powered Motorola Razr feature phone on Sprint, it serves a downloadable 3GP video file delivered via HTTP. Thwapr knows the pixel resolution of your phone’s screen and whether or not it’s a touchscreen. We resize the video to fit.”

The latest upgrade improves video quality as well. “If you shoot video on a BlackBerry,” Hoffert says, “it’ll play on some other phones with higher resolution than BlackBerry supports. On Android, we’ve just doubled our video quality. It looks great.”

Thwapr was founded in 2007 with funding of $3 million from angel investors. The company is headquartered in New York City and has 15 employees.