There has been some speculation that the show is cancelled but nothing definitive. In fact, the fact that the official bully beatdown facebook has been active lately is a good sign that MTV still wants to do another season. But at this point, since there has been no news of a casting call for summer, its unlikely that a season 4 would start this fall. Then of course there is the fact that Mayhem is now busy with UFC and may not have time to do season 4 anytime soon. And he has said that his contract with MTV is up, so he doesn't know if they will renew it.
http://www.youtube.com/watch?v=v_EvXbAdHFk (go to 7:45 and watch the rest, he talks about a possible season 4)