A forum is a social network, no? SNs just have some fancier php setup/js going on. In these avatar communities all they are is a forum with a super fancy avatar system, a shop for clothes to go with that system, and points (premium or not) to complement. All of the socializing is done on the forum side, which phpbb is very good at. Gaia is such a community and as I think you know it runs on modified phpbb2.
But I don't want a social network. I just want people to be able to customize their own profile's css so they could have a custom color scheme/background image or something. Think oldschool myspace. I'm unsure about moving blocks around... I think css will suffice for now.
I've seen this and its not quite what I'm looking for though I'll see if I can disable what I don't need, though I don't think itl do as far as css goes