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).