I came across a couple of studies by Sam Gosling and others who looked at personality as portrayed online, the first looked at personal websites and the second at social networking sites. Both links are to pdf files.
The studies both showed that the online personality displayed matched very closely the actual personality of the individual.
I find this really quite fascinating because it seems to fly in the face of perceived wisdom, that people seem “anonymous” online and that this is the reason for a great deal of the unpleasantness and flaming that you hear about (and fortunately in my case I’ve never experienced it). I would have thought if you can pick up a person’s personality, you would be unlikely to engage in the threatening and abusive commenting that does happen.