Shortly after Twitter announced that it acquired Vine, bringing looping videos to tweets, Facebook said it will block users' ability to search and find friends via the app. Facebook doesn't specifically spell out Vine in the developer update that affected the change. But it's hard to read the amendment as anything but a response to Twitter doing the same to Instagram after Facebook picked it up.
In a post about the change, Facebook’s developer blog justified it this way, “You may not use Facebook Platform to promote, or to export user data to, a product or service that replicates a core Facebook product or service without our permission.”
Basically, Facebook isn’t going to see a return on sharing its data with Vine, so why do it even if it makes the video app more difficult to use (and can’t look good for the social platform). Now, users will have to manually seek out friends who are using the app. Regardless, Vine has been getting widespread press and trending positively on Twitter for several days.
It's worth noting that even though Facebook is not allowing the app to access its data, users will still be able to share their Vine videos on Facebook.