It can lead to suicide. I've heard and seen many horror stories about how a loved one was bullied, literally to death.
It's very sad. I don't think people realize how much they can seriously break someone down on the internet. Just because it isn't face-to-face, doesn't mean it isn't hurtful.
"But its just a joke".
The person on the other end may not know that. It's hard to tell over a chat if someone is joking, or what kind of emotion they're meaning to send through that message.
Just because you may think something rude is okay to say, think about how it may affect another person.
Do you really want to be responsible for a death?
Responsible for ending someone's life just because you said something rude that you may not have actually meant?
Remember the phrase: "If you don't have anything nice to say, don't say anything at all." That goes along for messaging too.
Hope this helps, and sorry about my rant.
