One day, I logged into FB and it was suggesting my co-workers as friends.
Yah... they do that. B/c they are sleaze weasels. FB has... (pinky to mouth) one meeellllion ways to figure out your social graph.
You ever got lunch with a co-worker? Your phone and theirs were at the same table in the same restaurant. Boom. You know that person. Or a 3rd person adds both you as friends? Boom. Or someone took a photo and you and other person were both in it? Boom. Or they buy phone call data from a data broker? Your phone called their phone. Boom.
It's all but impossible to stop it. I never, NEVER, had a FB account in my whole life. I never have even loaded their page, and I block all their 3rd party scripts. I have a friend who also never had. I'm 100% sure FB knows we are friends. It maintains "shadow profiles" for ppl without accounts. Our mutual friends have linked us, and FB knows that. It still gathers enough data to link us. Despite we both did everything we could.