Great poll. I'm faintly surprised there are so few of "us" - people who use made-up names.
I created "Meredith" when I needed a username for the
dictionary.com forum . It was the name of a character I'd played in the computer game Ultima IV, and she was a paladin, so once I started emailing as Meredith the choice of surname was obvious.
As to why I use a false name - well, for several reasons. First, I have (or perhaps I should say had) a career in software development, and I didn't want my job to be linkable to my leisure activities. Not to mention I didn't much want my boss to see the timestamps on my forum posts.

Once I started writing fiction - and romantic fanfic, at that - and then nfic

- keeping it under a nom de plume became even more important.
Second, I chat about my kids online - not on this MB as a rule, but on IRC and on the dictionary forum. There's no way I'd want what I might say to be traceable to them - and even though they don't share my surname, my own name is extremely uncommon, especially in the UK. Last time I did a web search I turned up just one other person with my name, a scientist in the Netherlands.
Third... reading what Julie wrote, I realised I actually quite dislike my own name, too.

Whereas I really like the name Meredith and, well, I picked it all for myself, y'know?
FoLCs who've met me in person know my real name, but they call me Mere, which suits me fine. I answer to either name in person, and I use my "meredith" email account far more than my others. Though I discovered last year that I still have difficulty using "Meredith" on the phone - go figure.
I actually have a couple of other nicks (and a mailbox) that I use for various private activities - one for discussing health issues on a semi-public forum, and one, barely a week old, for discussing Home Education issues on a mailing list where there are UK government functionaries present. I don't *think* that makes me paranoid... does it?

Neither of those nicks has an actual identity associated with it the way Meredith Knight does.
Just thinking/writing about it this way makes me feel (and probably sound) schizophrenic. Really, it isn't a big deal. Meredith Knight is just me with a different hat on.
Mere

(recently christened "Mad-Eye Mum" by said kids)