Lovely to "meet" you, Lauri; my name is Robert Redford, and tomorrow I'll decide to be Ernest Borgnine
My real name is Matt... or IS it?
Granted, you can make up any name, even on Facebook. BUT, once you add bunch of family and friends to that Facebook and have to tie that to forum posts, that kind of possible exposure might temper your comments a little. Although that doesn't stop all the famous people from looking like morons on Twitter.
In the end is my original point. Being able to express oneself without your identity being exposed allows people to vent their worst selves. Before the internet, people could only be *******s with anonymous letters and wearing masks on Halloween.