This is very interesting. I also never fill these sections out (or the offline equivalent) if I can help it. Sometimes I'll put one or two things there, just to not seem like a total creeper. I try to avoid any personal information. Part of it is intentional -- less is more when the purpose of those types of sites is for people interested in you to stalk you, when really I want them to have to go to the source to get what they're really looking for.
I'm actually going through a period of particular ambiguity in my personality. When I'm actively engaged in something, it's easy to sort of define myself with whatever I'm doing (like defining myself as a diver). It helps me to function to be able to think of myself in a particular role -- I'm so-and-so's plus-one, I'm in charge of this Acme project, I'm X's mentor, or whatever. Thinking that way helps me to focus on the performance. Have you ever seen a television show in which one of the actors seems to have forgotten he's on screen? And drops character? I've been caught doing that a few times.
Even when people are naturally attracted to "me," i.e. I have not intentionally targeted with a version of me tailor made to them, it's hard to know what exactly that means. Is it my strength? My humor? My solicitousness? Unflagging support? If I don't know what it is they like about me, I don't know what to keep doing. It can be very disconcerting. I feel like I'm being interviewed for a job and I'm not really sure what all the job entails.
At times like these I feel like an engine with the clutch disengaged. I am nothing, but potentially anything. Like a discus, I could be sent me off in any direction, but ultimately it doesn't feel like it matters where I go or where I came from. I guess this is freedom. It also makes me a total anti-consumer. I don't feel at all defined by my belongings or my socio economic status. It's nice to run in the rat race only whenever I feel like it, not because my successes/money define me. But I also can't really force myself to do things I don't want to do.
Here's a BPD blogger (and SW reader) describing a similar thing for borderline personality disorder.