i'm in your conscience
I rarely use the same name on different sites. I think my most common username is Seiphre, which I use on maybe...four forums? PHO, Bulbagarden, 4Play, and Ninbuzz. I only use Sydian here, I'm sure. It was originally used on a forum that a friend asked me to mod. After that forum failed, we went to another forum run by a fellow mod from the failed one. There I was an Assistant Admin and used many names since I could change it when I wanted. I used Irishaboo, Genocide, Kurt Cobain, Color Me Happy, and Healthy Libido. I was Kurt Cobain on an everything forum as well, and Lemon Fences on another one...Bellossom or something.
Here I've only used SilverSmeargleSplatter and Sydian. I have an alt named Petilil. Speaking of that little green bulb, I am Petilil on Instagram, seedbby on Twitter, and seedbbychurine on Tumblr. my god.
Then Neopets, I've had pokemon_and_yugioh, button_chomper, killme__kissme, klareenet, iloverupertgrint7766, and schizophrenicfox. Out of all those, P_and_Y was my first, though I don't consider my first internet identity as that, because I wasn't allowed to post at all. My first true internet identity was iloverupertgrint7766. Bahahaha.
I know I'm forgetting some usernames in the mix...I have to be. lol But that's pretty much all of them. My favorites are Sydian and Seiphre, and I have a few others in my head, but if I told you, you'd go out and use them. ;) I dislike when people purposely take someone's username though. If it's an accident, I can understand (Aerilyn was once Sydian for two days, I kid you not), but don't take someone else's name. Be creative and get your own! :P