I was very (healthily IMO) paranoid about F/book before this.
I already knew that anyone who can see any pictures you put up, can copy them/screen grab them - obviously without your knowledge let alone consent.
I already knew that once you put photos up there, you don't own the copyright any more. They do.
I already knew never to let them into your email account - whichever one you pick to register with.
I already knew that I had to "friend" my own relatives and wait for them to accept the "friend".
What I didn't know, however, apart from this appalling demonstration to me today that they somehow know about my non-F/book Yahoo accounts, was that when you browse for people - relatives in my case - they offer you complete strangers to "friend" who are on your relative's lists. Complete strangers. Never heard of them.
But F/book knows that someone you're searching for, knows them.
Why the F would I want to contact a complete stranger just because a cousin of mine knows him somehow?
The whole place gave me the heebyjeebies. Even closing the account takes some effort - 3 different popup windows, multiple requests for password, AND typing in the random-generated words back to them, and multiple attempts to make you say why. I lacked the nerve to tell them to eff off.
