Earlier quoted context omitted.
I don't trust any of them. That's why I don't have a Facebook page, I don't have a MySpace page, I don't use gmail or any of Google's services. I don't have a Twitter account. I do have an account on LinkedIn because my boss at my last job asked me to set one up, but I never update it.
I understand where you are coming from re: privacy issues, but aren't you also missing some real value? I check Facebook 2 or 3 times a week to see what family and friends have been doing. With Twitter, I follow people into the same technologies that I am interested in, and I get a lot of interesting links. re: gmail: I have never bought anything from a sponsored search link, but I do occasionally buy things from adv…
But I prefer to keep up with friends and family either in person, or via phone or email, and I don't update my own page more than once every few weeks.
I found Twitter to be far too expensive (in terms of wasted time) to be worth bothering with. It's not just the necessarily regular checking for new tweets, but the fact that this distracts me and breaks the flow of whatever I am supposed to be doing. I would guess that twitter users waste many hours per week (and possibly per day for heavy users) like this, in exchange for relatively few insightful comments or useful links.
I think it plays off people's desire for instant gratification, but the opportunity cost of using it is very high.