If you have your Facebook settings to "friends only", then only your friends can see your posts. That is, only people that have contacted you, and that you have approved.
Total strangers can see some stuff on your Facebook page. They can see your name (which kind'a makes sense, because how else would people be able to send a friend request?). They can see your profile photo (your face, if you posted it). Again, this makes sense as there are a lot of people with your name, and being able to see a face with a name helps people to recognize you. They will be able to see if you filled in where you live (the city), and where you're from (again, just the city). They can also see your friends.
This amount of information, which is the bare minimum to allow people to figure out if it's really you on Facebook, is what most people will give out freely in public in real life. They will show their real face (unless they're going to a masquerade party of course) and they will give their real name. If they mention that they live nearby, people can get a pretty good guess what city they live in. And if they allow their friends to hang around with them, people will know who their friends are.
I always recommend to everyone to have their Facebook page public. That way you aren't tempted to say nasty things (which you really shouldn't say in public - and Facebook is a public place). And you have to ask yourself what harm people can do to you if they find out, for example, that you own a dachshund, or volunteer for an Historical Society.
I'm a big believer in personal and public integrity. I'm a believer that you can make the world better by speaking up, in public, in a responsible way. Every word you speak should build up what people think of you. If you're saying things that you'd really rather not get back to you, I recommend that you don't do that. If you're a nasty, gossipy, kind of person, I'm gonna recommend that you stop it. And if you're a really nasty person, I'm gonna recommend that you stay away from Facebook, and just stay away from everyone else in public, too.
By the way, my Facebook profile is public, as is my Google+ profile. Go ahead and check me out, you'll also find that I draw cartoons. Thanks for letting me speak up.