• by Gavin O'Malley  @mp_gavin, April 1, 2015

Continuing to experiment at the intersection of mobile and video, Facebook is rolling out Riff -- a new app that lets friends collaborate on the creation short-form video.

Once Riff users share a video, their friends are then encouraged to add to it any way they like. Making things more interesting, friends of the original video creator's friends can then contribute to the video.

Coming out of its Creative Labs, Riff is part of Facebook’s app diversification game plan. The social giant is widely believed to be pursuing this strategy in order to create additional revenue streams and increase overall mobile usage rates.

“Our hunch was that if you could make videos collaboratively, the creative process would be more fun and the final product would be cooler,” Josh Miller, product manager at Facebook, explains in a new blog post.

“The potential pool of creative collaborators can grow exponentially from there, so a short video can become an inventive project between circles of friends that you can share to Facebook, or anywhere on the Internet, at any time,” he adds.

At present, video and mobile represent the biggest growth engines for Facebook. Indeed, the amount of video shared by users and brands increased more than 300% last year, while mobile now represents a whopping 69% of total ad revenue.

With the launch of Riff, Facebook is also fighting for relevance among young users, many of whom are spending more time with new mobile apps like Snapchat and Yik Yak. Facebook’s domestic rate of growth appears to be leveling off, according to the latest survey of social media use by the Pew Research Center.

Last year, CEO Mark Zuckerberg used a “town hall” meeting to address Facebook’s app strategy. “On mobile, each app can really focus on doing one thing well, we think, and the primary focus of the Facebook app, today, is News Feed.”

Not unlike Riff, Facebook already offers Slingshot -- a Snapchat-like app that allows users to share pictures and video along with text to friends.