A social cannibal is a person, usually a friend, who is a friend when it's just you (and other friends in your group, this can have multiple victims at once but is not as effective the more friends he cannibalizes at once) around but as soon as he's around other people, friend or not, if he needs to he will feed off a friend's or multiple friends' faults to better himself socially. Usually it involves heavily over exaggerating stuff about said friends. The cannibal will usually deny doing this. It also counts if the person changes opinions and views on things based off the person they're around, things that you both agree with that he can turn around and accuse you of agreeing with that opinion.
Bi-stander: Yeah so people who play guitar are so gay.
Cannibal: Yeah! You're right! I hate guitar players
Friend: But dude, we both PLAY gui-
Cannibal: Uh, dude I used to but then I saw how stupid guitar is.
Friend: But dude just yesterday you where saying how you love g-
Cannibal: I didn't say that! By that I mean't I actually hate it.
Friend: dude you're SUCH a social cannibal.
Cannibal: no I'm not dude? what do you mean?