Of course it isn't completely anonymous; Facebook isn't claiming that it is. Their documentation very clearly demonstrates that they are linking the "anonymous"

Of course it isn't completely anonymous; Facebook isn't claiming that it is. Their documentation very clearly demonstrates that they are linking the "anonymous" user with a Facebook profile on the backened (evidenced by the ability to connect accounts in the future). It is anonymous to developers because they don't see the profile information until the user decides to link it. This isn't some service being offered by Facebook where we have to then worry about them "connecting the dots" in the future - they already told us what they're doing and how.