Someone whose opinionated, volatile, easily offended, insecure nature makes them super-easy targets.
Bieber/Twilight fangirls are easy troll bait because they take ANY negative remarks about their "god" as a personal attack and will violently attack expecting the trolls to stop and make themselves stand in the corner but their attacks never cease, troll feeding ensues.
Person 1. Hey dude where are you going for lunch?
Person 2. To Taco Bell!
Person 1. You'd better hold on tight to your bag, Taco Bell is the most powerful BEANER BAIT! " Yo Quiero Taco Bell".