Even after deleting FB cookies, what prevents them from tracking you (with reasonably good accuracy) using your IP address. In that case, you might as well just blacklist all of facebook.com.
In my opinion, internet users must be aware that there is no easy way to be totally anonymous, whether it be Facebook, Google, etc. If you require complete anonymity, you might as well unplug your internet cable.
what prevents them from tracking you (with reasonably good accuracy) using your IP address
Dynamic IP addresses and use of the same IP address by multiple people.
All it takes is a couple of friends, acquaintances or others accessing using your home network and it'll confuse the hell out of the stats. And that's without going into IP ranges for universities, schools, offices large and small, and your local coffee shop.
Then add in IP address pooling by ISPs, where every time a user connects (or every week, month or year) they're issued a new IP, and you end up with an unclear situation.
I won't start on how cell/mobile phone networks further confuse the situation ;)