I am using Chrome and this just happens in Facebook and just for YouTube videos (HTML5).
When going fullscreen it shows the video in for a second and than the video disappears and I see the Facebook page in fullscreen with the "www.facebook.com is in now full screen. Exit full screen ESC" message on top. I can click links but the page will not scroll. Pressing ESC exit the fullscreen mode and I can continue watch the video in the standard size.
When opening the same video on the Youtube page there is no problem to go fullscreen.
No problem watching Youtube videos in fullscreen from other sites.
Has anyone else experienced this problem and found a solution?