I really really don't like being called anything but my name, so I try to use my names(especially my first name) as my username.
If I can't use one of my names as my username at any particular site I almost never register there, or if I do I might register a name just to ask a single question using a throwaway account, but it doesn't feel right to continue using the account because it's not actually my name. There have been many sites about subjects I've been interested in but which I haven't registered for because I couldn't use my name/names as my username.
I know this is a security problem, and I would really like to use a different name for security reasons, but I still have trouble with being called anything but my actual name by another person. Several times I have sat for literally hours just staring at Gmail trying to come up with a username I'd be comfortable being called but I haven't come up with one. I am comfortable coming up with a random string of characters to identify myself to a computer(like for a bank's web site), but if there's going to be any communication from other people I feel a need to use my actual name.
A few years ago I stopped using the combination of my first and last name as a username because I didn't want it to personally identify me, and I've been trying recently to stop using my last name(although it doesn't uniquely identify me), but the name "matt" is very common and so is often already taken.