"online social networks use third-party tracking sites, called aggregators"

For Firefox users, there is now a new addon called "BetterPrivacy" that blocks something called 'LSO's - Longterm Storage Objects (sorta like cookies, used to track your site visiting patterns) - which are implemented in Flash - and cannot even be found unless you have the latest tools like this...

There is also "TACO" (been around for a while) which gives you a pile of 'opt-out' cookies for your machine that are supposed to be respected by many web sites now.

Robin