If you have any friends in common with anyone else, they will bve getting the procession of "People you may know" too. Some of your friends may have a very wide circle of people they hardly know themselves, if your circles overlap you are visible to their friends, whoever they might be.
Also if you post anything on Facebook, your name appears on it and a click on that goes to your home page.
I think some "US generals" just trawl Facebook for likely posts, click on the name of the poster to go to their home page, then send a friendship request to anyone who takes their eye. If even one in a hundred responds, they have one person to keep up a correspondence with.