Most(?) sites that offer a FB login do just that – offer a FB login. I can’t offhand think of any where it’s mandatory.
It’s coming. Gawker Media (Lifehacker, Gizmodo, Jezebel, io9, Deadspin, Kotaku etc) recently made it compulsory for commenters to log in with a Facebook, Google or Twitter account.
Interesting, I get a fair number of those via Flipboard, but I can’t remember if I used my Fb account as a login.
Flickr do now, which I’m pleased about, as I used to have no-end of problems trying to login using Yahoo; I’d made a bit of a hash with BTYahoo and Yahoo accounts, and really struggled to login, Flickr kept booting me back to login pages.
An exercise in tooth-gnashing frustration.
Tried it on my iPad using Fb, boom! Straight in, no issues, going to start using it again now.