Somebody who constantly writes on someone else's formspring, anonymously or not. Typically incredibly annoying and leads to clogged feed.
Anonymous clingers mostly turn out to be immature teens who live their life on the internet and have few or no friends in real life.
Bill: Dude, I got like 400 questions on Formspring from Suzie yesterday!
Bob: Uh-oh, you've got a Formspring clinger. Commence evasive maneuvers.