I'm on FB, but used to be on netlog too. I didn't combine the two for long, just didn't want to.
I'm not on Twitter, don't really care about it for now.
I don't take FB too seriously. It's a useful device to keep contact with many people without having to do the effort of calling, mailing or visiting them constantly. Its a good medium to keep any degree of contact, from constant interaction to "meh, just funny to know what he's doing these days". But I'm never on it when I'm not on my computer and I don't have to visit it everyday (and actually never when I'm on vacation).
I don't post too much, especially not about what I'm doing or pics from my normal life, but I must say it's sometimes tempting to do so (just to cause reactions). And with some random "voyeurism" and fb chat I can spend hours a day if I'm not working.