There are other options, e.g. proximity. Or your colleagues have added your email or phone number to their contacts which messenger/facebook can access. It's a creepy feature but Linkedin selling their network graph is too far fetched in my opinion.
I thought about it. But it gets creepier. Former colleagues in different geographical locations who don't have my phone number/contact info but are only connected on linkedin appear in Facebook.
Does your company funnel every location through the same internet connection? And some people put their employer's name on Facebook. They could be determining from your colleagues in close proximity which company you work for and then connect you with any others they are aware of. Still creepy but perhaps not as scary.