My friend's computer plays YouTube videos fine, but on Facebook the videos show only momentary flashes of the images. (the audio plays ok though).
I had her uninstall/reinstall flashplayer, did not make any difference.
Also she says that on the YouTube homepage, that ad at the top (today it is for Final Destination5) does that same flashing thing, while the other YouTube videos all play fine.
She is running IE9.