A facebook email/wall post written to you from a trap requesting that you see them in a social setting. A kind friend will usually comment on such a post with the words 'posting in trap thread' to remind recipient that they have been targeted by a trap.
Trap: Hey! We should totally do something friday nite!
Recipient of trap message: ok
Person 1#: Posting in trap thread
Person 2#: IT'S A TRAP