I've seen this like a hundred times, some chick's sucking a cock and someone comments "OM NOM NOM", someone's eating a chicken leg, "OM NOM NOM", someone just plain has their mouth open, "OM NOM NOM", etc.
Memes are for 4chan, I'm personally getting sick of seeing "OM NOM NOM" every time I check the latest comments.
edit: example at post #310924
Updated by a moderator