Facebook has a real names policy, rather sketchily enforced.
There are a number of serious, practical reasons to oppose a real names policy-- there are people who need privacy for their safety or to not lose their jobs (I wonder what happened to Ahashuerus(sp?) from rec.arts.sf.written) and there are people who've built a social network around a nickname and losing that network is a serious thing for them.
However, what gets lost in those discussions is the positive argument for nicknames. Nicknames are fun-- they're a realm of playful invention and flexible identity. I don't want that to be lost. Besides, I mostly find nicknames easier to remember. They're generally more varied than real names and they have more emotional resonance.
I don't use a nickname because I assumed that if there was a right nickname for me, it would be obvious to me, and that never happened. (I don't consider nancylebov to be a nickname for Nancy Lebovitz-- it's just a desperate effort to have something a little easier to spell.) Besides, anonymity would be hard-- I've had more than one person say that they hear my voice when they read my writing.
I recommend Two Cheers for Anarchism: Six Easy Pieces on Autonomy, Dignity, and Meaningful Work and Play by James Scott, who started his career of mistrust of centralized authority with research into governments regularizing names. (He also started with research into limitations on nomadism, but that's another large topic.)
First link thanks to supergee. Which reminds me-- Live Journal and Dreamwidth are very usable social sites that *don't* have real name policies at all.
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