My phone has been doing the same thing.  Today, I had enough and did some searching online with no luck on solutions that are actually feasible (root phone, complete reset, etc.).  Since it just appeared to be YouTube videos I was having a proble with, I went to Play Store and was going to unistall and reinstall YouTube app.  Did a search for "youtube" and found the app. When I clicked on it, it showed the usual screen but said "disable" where it normally shows "install, unistall, open".  I clicked on "disable" which restored itself, refreshed the screen and the "uninstall, open" boxes reappeared.  I then went back into facebook and clicked on a video link and was finally able to get the videos to play again.  Have NO idea how or why I would have disabled YouTube... but the videos are working again.  Hope this helps!!!