They use the local network environment as yet another input into the tracking/stalking algorithm to both track you and also track others (let's say you are on the same network as someone else and the social network doesn't yet know the association, based on network data being the same the social network can infer a connection between both of you).
I'd like to know too. On Wireshark I saw that Facebook and Amazon are both involved in my computer somehow despite my never using those services. So I too want ab answer to this question.
Mark