It's a punctuation mark that ignorant people would rather not use. Seems like they'd rather use an ellipsis (...), which is wrong.
susyq: hehehehe so at my birthday party today joe walked over to me and told me that im cute. neway i told him i dont like him and he should die. i probly shouldnt have but i can do whatever i want...afterall, its my party and i can do what i wanna.
Joe: DIE LIKE FUCK. By the way, you should have used a dash there instead of an ellipsis. Now excuse me while I cry my eyes out and go emogoth—that is, a person who should probably just die already, rather than cause the world to suffer their presence any longer.