Your IP address, browser fingerprinting (despite the private browsing) and so on.

There are just too many ways to identify you, and therein lies the problem.

If this kind of thing is concerning to you, you have no option but to delete the FB account.

EDIT 1: Removed MAC address from my comment, which was incorrect as noted below.

EDIT 2: This is a test that is available from EFF to test your browser fingerprinting: https://panopticlick.eff.org/