A person who tries to "fit in" with a clique/group that they do not belong in. They cling unto you and won't go away easily, hence the name leech. A social leech follows the clique/group wherever they go, even if they don't pay any attention to him/her and give hints for the leech to leave. Most social leeches worship preps and will buy preppy clothes in a desperation to fit in. Similar to a poser, but more annoying.
Prep 1: Omigod, look at mandy. Why is she hanging around us?
Prep 2: Man, shes such a social leech. I wish that hoe would leave