Platforms like Instagram's Hyperlapse, Vine and Snapchat all have some sort of combination social features and video, but the new Riff app from Facebook takes video sharing to the next level.
Riff is a standalone app for iOS and Android that lets its users create short videos up to 20 seconds in length. Once these are finished, friends can record their own responses and add them to the original. There are no limits to how many people can tack on their own videos, so the idea is that the compilations become even bigger and better each time they are watched. Rather than having people be passive consumers of social media content, Facebook's Riff makes it easy for anyone to become part of the creative dialogue.