1: Most (if not all) major social networking sites are run by a business.
2: All businesses need to make money. Even non-profit businesses need to cover the bills.
3: If you aren't paying directly, someone else is.
Thus, like (at least broadcast) commercial television and radio, you - the viewer, listener, social networker - is the product that they are delivering to someone, usually advertisers.
But, as anyone whose even paid a bit of attention to the business discussions related to the internet over the last ten or so years knows, display ads on websites don't produce a lot of income. Thus any social network will either have to become a paid service (and loose members who don't think its fair to pay or cannot pay), or find other ways to use what they have to make money.
That said, there still are things you can do to protect whatever level of privacy you want to protect. If the site in question appears to be leaking more than you like, leave the site or use it with great care. If you can at least inform them that you don't want this kind of information shared.
I'm sure, since I do things like use grocery store (et. al.) loyalty cards, I'm considered someone who doesn't care about my privacy to some of the more virulent privacy advocates. I figure, at least in that case, I'm being reasonably compensated for the marketing information they are giving me.
The latter being said, I did make sure to opt out of having Facebook share anything - at least directly - to website and made sure that my friends could only share the stuff that is already pretty much public knowledge. And I figure if they could find it from my public profile, I don't care if Facebook (or LJ, Google or anyone else) sells it to them and profits.