I have indeed got a completely different person on the internet not just by my username, but my nicknames too. When online I'm much more social when it comes to talking to people and have a rather confusing explanation to everything, but Crissy (my most common nickname) is the one that seems to be the social bunny. Cold Fusion though is rather quiet and just tends to stick into the background and gives the wise words if needed. She generally has the intellectual side and the serious part. When my username was Eye Of The Storm I was rather grumpy all the time, irony much, but I tended to just let leash my anger more often than not with no explanation.
But, get me on MSN and I don't talk at all. At all. Well, unless I'm spoken to most of the time unless I'm in a really, really confident mood. But I noticed that depending what my name is on there, like right now it's set as "lovestruck;;" and I'm much more hyper than normal. The only time I speak to some-one first is if I know them well most of the time but, even then I can be like that.
Offline though I tend to be more like Cold Fusion in a way with a good old whisk of Crissy when it comes to family life, but I don't at all have any social in me, I tend to just stay out the way of almost everyone. Online I tend to feel as though I am more myself that I am offline so if anyone met my offline self they'd probably have a closer friendship with my online self than my offline self.