There's a theory circulating that some victims of online bullying might be engaging in a form of self harm, by deliberately exposing themselves to situations in which they know they will be victimised.
Most bullying victims would be able to do something to make the bullying end. But many times, the victim doesn't have this knowledge or is for some other reason able to defend him/herself. I was bullied throughout middle school, but it stopped when I got the social finesse to defuse it. If I had kept on the way I was acting before, I would have become a victim of bullying at my workplace when I was 23. I've heard similar stories from others. You're not the source of the bullying, but there are different social mechanisms which can make you a very tempting target.
Like all difficult interpersonal problems this needs to be attacked from multiple angles. Just wanted to emphasize that bullying is not at its core a form of self-harm.
I'm pretty sure I went through this in my teenage years, but whether this is a more common problem to the victims of bullying -- that I'm uncertain about.