This seems to be an effort by Facebook to increase user engagement and possibly generate video ads in the future.
How Riff works
The use process starts by creating a topic on Riff and posting a video which is then shared with the user's friends. They can view it and in turn choose to add their own clips on that topic after logging in using Facebook ID.
In this way, a short video can become an inventive project between circles of friends.
Videos have a max runtime of 20 seconds and can be fast forwarded if the video threads become lengthy.
Riff will compete with JumpCam, Vyclone, MixBit and Snapchat's Our Stories feature.