Facebook uses this: The Beacon online ad system--intrusive and evil!
Fortunately, I use the Firefox browser, and people who use Firefox can block Beacon (and a lot more besides). Read the whole thing, or, if you just want to know how to block the hungry hungry corporate hippos, scroll about halfway down.
When I get home from work today, you'd better believe I'll be blocking the fuck out of Beacon and sending those Facebook people a strongly worded email. Why I oughta . . .