A like spammer uses the Facebook Like Button on a person unrelentingly on all of their pictures, comments, notes and whatever other crap they have posted on fbook.
The act has no monetary gain and only exhibits the like spammers true lack of a life and friends. It is done for teh lulz and the common response is ffffuuuu.
Storian: Haha, I'm gonna like spam Leo!
Hector: Yes! Make sure to get all his emo statuses and stupid FarmVille posts!!! FTW ZOMG