I was recently at starbucks, and they list the employees aka baristas first and last names on the bulletin board. There is this cute girl so bottom line is that I DID CHECK facebook and she was one of those to not have her facebook privacy settings tweaked and I could see she had a boyfriend "in a relationship with ------ ----" etc. I could even view her hometown, her high school, hobbies, etc and then I could go to wikipedia and greatschools.net to determine the socioeconomic status and public schools of people in her hometown hahah I do this a lot!!! Greatschools.net shows the percentage of a high schools students on free or reduced priced lunch. Some high schools it's 1percent. Some inner cities it's 70plus percent. This is off topic but it could give you insight as to whether she would be a gold digger or something like that.
If the privacy settings prevent this kind of info, perhaps you can tell from a profile preview pic when you search for them...we do this all the time, If in her profile pic she is hugging/whatever with one guy in the pic, then she's probably taken.
People do this all the time and it's like being a fly on the wall and the victim doesn't know your presence. Like that song by Clay Aiken "if I was invisible"
It's why I would advise everyone out there (both guys and girls) to tweak social network/facebook privacy settings!!!!
If you're not going to adjust your privacy settings, ok, but realize whatever you put is like you're just saying to the world "LOOK at me and my info!" Which is fine but your profile will get a lot of visits, esp if you're attractive