This critic generally dislikes documentaries that leave the audience wondering which parts were real and which were fake. (Paper Heart with Charlene Yi and Michael Cera is Exhibit A.) But, that conceit -- what can you believe and what can't you? -- is put to brilliant use in Catfish. You may not be sure what to believe at the end, but I think that's precisely the point.
Catfish is about people not being who they say they are. Especially online. It's a mystery and a drama and a cautionary tale. It leaves you wondering and doubting.
This critic also dislikes spoilers, so she's not going to say much more than this is a movie for the facebook generation. It's intriguing, sad, pathetic and thought-provoking. See it.