Catfish is a movie that explores the consequences of meeting people online - and in this case, on Facebook.
Everyone has their own suspicions about people they meet online, but today it is very common that people make more friends through communities or games (like World of Warcraft) that they become very close to, and though some never meet in real life they are still considered better friends than the ones they meet daily.
So Catfish pretty much deals with the subject of meeting someone online, becoming very close, then close enough that the people involved start falling for each other and decide to meet in real life and start dating. See the movie for yourself to see what actually happens.
Trailer for Catfish: http://www.youtube.com/watch?v=BuE98oeL-e0