My name is Mike Patterson. That's almost what it says on my birth certificate; it actually says Michael. It also says Michael on my university ID card. There's some other very similar variants on pieces of government ID, some include my middle initial or full name. Few people use Michael and fewer still even have reason to know my middle name. I've no real reason to use any other name, save for convenience - both my own, and for other people. Mike, as you might know, is a pretty common name, particularly amongst North American males, and doubly so amongst those of my generation.
Depending on context, I have a lot of other names to which I might respond. Depending on the context, I might also not respond, as they tend to be, well, contextual.
At work I'm usually just Mike, but I might be mpatters. That used to be my email address and it will still work, but I now tend to publish mike.patterson. I also use that for email addresses elsewhere, sometimes; my alumni forwarding account, for instance. Since my name is published in our directory as Michael though, some people do call me that, and I don't bother correcting them. I might not immediately respond though.
On Twitter, I'm snowcrashmike, but nobody uses that anywhere else. Some very old IRC hands might know me as kraig, and I used that on MUDs. I also go by kraig on LiveJournal and Dreamwidth, and some people who met me first through those communities tend to use it as my name, even in voice conversations. In some web communities and games (Forumwarz, for example) I'm generally kraig or kraigu, but sometimes kraigus.
In the very long ago, I used to use BlackSpy as a BBS handle, but that was too overloaded when I switched to IRC. Nowadays on IRC networks I tend to use kraigu. My Steam name varies, I usually set it to earless wondercat, but I often play with it: earless wunderkatze, earless dyingcat (I do a lot of dying in online games), fearless wondercat, feared blundercat, beerless wondercat… well, you get the idea. But usually people call me just earless or wondercat in in-game voice chat, it makes things easier.
Sometimes, in certain circles, I might be just [. I don't even remember what my FIDONet handle was.
I'm old enough that things like IRC were fairly new when I was coming of age, and young enough that identity is really a fairly slippery thing. I'm old enough to still value some privacy and think that for some people it matters quite a lot, young enough to realise that not using your real name doesn't mean you can't be found. I'm experienced enough to know that a middle ground is very difficult to find, wordly enough to know that different cultures approach "true names" in vastly different ways, and finally, practical enough to not really care what name it is that people actually prefer - I try to find out what it is, and use that.
I think realnameonly policies are unnecessarily invasive, don't do what proponents claim they do, and insensitive. In short, I think they're misguided at best, generally no better than security theatre, and at worst, just flat-out stupid. Nobody else has the right to tell me who I am, nor do they have the right to tell anybody else who they should be.