Would it be possible that he googled you, or you googled him after the meeting, that one of you searched for the others' LinkedIn or other similar sites (or on FB itself), and that the Facebook cookies/trackers connected the two of you?
Disclaimer: I am not connected to the tech world professionally; I'm just a bystander.
Step 1. Read Data and Goliath by Bruce Schneier. Step 2. Delete your Facebook account.