The idea is to move towards "logging in to the Internet" instead of logging in to each site. In another version, your browser handles the central log in.
Your public profile is public anyway. Your Facebook friends list can probably be seen by anyone who is logged in (and anyone can register and log in, so that's anyone able to see your friends list).
OK, they can't get your email address, but, if you don't log in using Facebook and still want to log in, you're going to have to register on the other site and give them your email address anyway.
It's not the information as such that gives me pause for thought, so much as the linking together of everything. If I'm using Facebook to log in to various other sites, that's me definitely identified as the same person on all of them.
I just use a password manager and have a separate account on each site.