It depends on their privacy settings. Some people expose a lot of information, and you can (if you're pateitnt) put together a great deal of information about a person with their privacy settings locked down if you study enough of their friends.
Of course, FB could just resort to making everyone's info private. But then it would suffer a serious loss of utility, as many friendships and social connections are validated by the existence of mutual acquaintances. Most of the time this is completely innocent and desirable for all parties, which is what allowed FB to become so popular in the first place.