Just a little about my experiences:

I opened a Facebook account last year after reading about it here on Mudcat (and one of the first things I noticed was that practically every third-level student in Ireland must be on it!). I didn't use my real name or DoB or even email address, just as I didn't when first going on to MySpace, YouTube or LibraryThing.

After getting to know MySpace I found it to be a great resource for music, and started using my real name.

It took me longer to get adjusted to Facebook, as at the beginning I was bombarded with people wanting to play those (IMHO, childish) games, but recently I've discovered great information and sources for library work and web development that I do, and I may start using my own name.

YouTube is a fantastic resource for music and documentaries (I'm addicted!), but I don't think I need to divulge my identity there.

I'm just being cautious in not identifying myself, and think most people should be equally so, but the rewards of joining these sites can be immense.