No, Facebook isn't done experimenting with mobile apps yet -- meet Riff, a just-launched app for Android and iOS that has you building collaborative videos. You start by recording a short clip (under 20 seconds) and giving it a topic. After that, your friends can tack on their own videos based around that theme. For example, you can start a "what I do for a living" compilation to find out what all your friends are working on. There's definitely the potential for pranksters to ruin the whole thing, but Facebook lets you either delete individual clips (if you're the creator) or report a whole video if it's going horribly off-track.