I found some tips to help stop social networking sites from tracking you outside of them at a surprising place: Infoworld.
And yes, to one degree or another, social networking sites can track what you’re doing on the rest of the web. If that doesn’t bother you, move along. If it bothers you, read on.
The main thing they can track is clicks. Any time someone posts a link to anything and you click on it, those sites can track whether you clicked, and how long you stayed there. If you don’t like that, copy the URL and paste it into your location bar rather than clicking on it.
You can also use the Firefox extension trackmenot, which fakes search queries into Google and other engines. If you can’t stop them from tracking what you search for, at least you can obscure it by burying it in a bunch of bogus searches. Another addon, called Better Privacy, eliminates some of the so-called “super cookies” used for tracking. I’ve been using the latter for the last couple of years.
And the last suggestion in the article was to use a different web browser, such as Google Chrome, exclusively for social networking, while using another browser, such as Firefox, for everything else. That should work, but I would recommend installing Better Privacy in both browsers if you want that tactic to be effective. I believe it’s possible to use a Flash-based super cookie to spy on what’s going on in between browsers. And if it isn’t possible today, someone’s working on it.