From time to time on Facebook I get suggestions for friends, where I think “How the hell did they know about that person?”, i.e. I do actually know the person, but can’t figure out how Facebook would have been apprised of that.
I’m surely late to the game, but in doing some poking around on the web, I was made to realize that anyone with whom I’ve exchanged email in the past might have run the “search my contacts” feature Facebook has… and their contacts would contain me either because they’d added me or because email programs tend to auto-capture anyone in your From/To/CC lines.
Like any bit of data on the internet, once it’s out there, it stays out there forever and spreads slowly across the universe…