Socialcam, which grew over the last year to become Facebook's most popular app, has parlayed that success into a $60 million sale to Autodesk, a firm known best for designing professional software for architects and designers. It's a strange but smart fit. Autodesk has lots of revenue from its business clients, but is eager to expand its user base to the average consumer and evolve from the desktop to mobile. Socialcam, a sort of Instagram for video, has tons of users engaged on smartphones, but no revenue and slowing growth.

We've written before about the roller-coaster ride a number of Facebook apps went on after integrating with Facebook's Open Graph. Socialcam and Viddy were two of the most prominent examples. Both added tens of millions of users in the span of just a few weeks. But this kind of viral growth, referred to by venture capitalists as "startup steroids," is difficult to sustain. Socialcam is still the number one app on Facebook, but has seen its monthly active user base shrink from a peak of around 80 million to about 54 million over the last month.

SocialCam was an in-house project that spun off from Justin.TV. The company has just four staffers and has taken limited angel funding, so a $60 million purchase is a big win for the small group of employees and investors. For Autodesk, which has more than $2 billion in revenue in and almost as much in the bank, it's a drop in the bucket. The key now will be getting some of Socialcam's numerous young users to stick around and try out some of Autodesk's pay services in 3D modeling, photo editing, painting and drawing.