To be honest I have never worked with socket.io, but I have worked with sockjs. I don't know what are the implementation differences, but sockjs also has each connection identified by its own ID (which is a UUID, as far as I see it). But the ID changes every time a user reconnects, so I believe it is not a useful way to identify users.
Maybe you should store some fixed user identification (such as email address, username) and relocate the user by that parameter (that's what I'm doing usually).