Developers: if you care about privacy-conscious users, you may want to fix that.
Regarding Facebook, we do require Facebook to login (as mentioned elsewhere, we're going to add other ways to log in soon) and we require the Facebook SDK for some of our social stuff. That said, we should handle this better - we shouldn't get stuck in a permanent loading loop!
Plenty of sites have a prominent Facebook sign-in button they steer you towards, but will reluctantly give you a username/password, if you insist. Songkick, for instance, or the very cool 2u.fm site that another HNer posted here recently. Others accept Facebook or Twitter, or Facebook or Google.
But Facebook-only seems pretty uncommon. All I can think of are TechCrunch comments, and the Scoble-beloved Highlight app for iOS. Most developers seem to avoid going Facebook-only because they face a backlash (as the devs behind this site have acknowledged already).