I think you may not be aware of how Facebook "Likes" work and how the can be evaluated _if_ you have a Facebook account. If you _don't_ have an account, you're quite right; then it's about as dangerous as what I'm doing (i.e. using the same username on several sites in the internet).
The problem with Facebook isn't that it's the most popular social networking site. I don't have anything against social networking sites per se and I'm even a member in one. The problem is how Facebook in particular treats your data. As a default you're wide open to the world, and while there are (horribly complicated) ways to _limit_ the amount of information that is available about you, the average user is either unaware of them or doesn't properly understand them (and, as ffaristocrat so aptly proves, sometimes also simply doesn't care).
One of the most striking problems is that everything you make accessible to your friends is also automatically available to all of your friend's Facebook apps. Essentially this means unless you don't make _any_ information available to them (or they don't use _any_ apps), your data is immediately known everywhere and by everyone.
Oh, and before I forget it: Don't take my word on any of this. Inform yourself, read up on Facebook and their security settings and make up your mind.