[d0mine]

People say if you don't want facebook to know anything about you, then you shouldn't post there. So others reply that it doesn't matter that you didn't give the data to facebook, one of your friends might.

So now the statement is that if you don't want facebook to know anything about you, then you shouldn't tell your friends, colleagues, etc. anything - after all, they may enter it on facebook.

</quote> http://slashdot.org/comments.pl?sid=2481922&cid=37750358

If you visit pages that might contain Like button then facebook will know about you even without your acquaintances.