I don't have a source for this right now, but I remember reading that the vast majority of disk space used by Facebook is taken up by users' photos. They are probably saving hundreds of uploaded photos per person (on average) and while they do compress them quite a bit this still has to take up a lot of storage space.
Of course they do also collect a lot of behavioral data, but I imagine all the data Facebook has collected on me probably uses up less storage than my profile pictures alone...
Also I remember reading that they keep 7 copies of everything at all time a few years back in an article about how deleting your account didn't actually delete your data, which became obvious after data from accounts deleted several years before had reappeared after the introduction of a new feature.
Of course they do also collect a lot of behavioral data, but I imagine all the data Facebook has collected on me probably uses up less storage than my profile pictures alone...