This is only true if your private information is visible to your friends. I don't feel compelled to put anything in facebook at all except the stuff I'd normally put in a public-viewable blog. My profile is 100% blank except for my profile picture, which is a monkey in a nice wool coat.
The only reason I actually use facebook at all is for party invites, if my friends could find a different way to wrangle party invitations I'd dump my account like it was myspace in 2007.
I'm curious, a lot of my information is snarky (location: Antarctica, religious views: Nihilist, etc) -- I wonder if they're able to realize that I'm obviously being intentionally inaccurate?
To put it another way, you don't have to talk to a person to 'know' them to varying degrees of certainty. You can gather bits of information and build your own image with out ever meeting.
The only reason I actually use facebook at all is for party invites, if my friends could find a different way to wrangle party invitations I'd dump my account like it was myspace in 2007.