I wish they would update this page to be more transparent. If they are in fact using LinkedIn's network (I just saw your comment below about co-workers which are not in the same geographical location), I hope they let users know and make a feature to opt-out of mixing it with Facebook.
Unfortunately, Facebook's official explanation of "People you may Know" is chalked up to "many other factors". So I don't blame you for speculating.
https://www.facebook.com/help/163810437015615
I wish they would update this page to be more transparent. If they are in fact using LinkedIn's network (I just saw your comment below about co-workers which are not in the same geographical location), I hope they let users know and make a feature to opt-out of mixing it with Facebook.