I just created a new Facebook account after maybe 4 years of radio silence. Two years ago, I had a job doing IT contracting; often I would go to businesses and repair laptops or run cable to a COM room. We had very very few residential clients since they weren't worth our time; the few that we did have were really just courtesy for doing business for so long. I went to one residents home a SINGLE time, hardly interacted with the man, and he definitely did not know my last name.
Guess who pops up on my "Suggested friends", with no mutual friends or place of work or any similar "liked" pages? Yeah, that one client.
Similarly, we worked in a small office in a cold storage facility, and Facebook also suggested that I add their accountant as my friend.
It's really creepy, but if Facebook was able to know that I worked at that employer then it's possible that it was able to make the connection.