Warning. Some movie spoilers. But you'll have been living under a rock to have not seen most of them already.
http://insidemovies.moviefone.com/2010/08/24/movies-that-would-have-been-ruined-by-facebook/
http://insidemovies.moviefone.com/2010/08/24/movies-that-would-have-been-ruined-by-facebook/