Earlier quoted context omitted.
All of the above. And also because, I just don't want to have to use FB to login to a totally unrelated site. The use case is, I use FB sparingly because I don't have time to get sucked into that pit. I friend people I meet irl, and 'Like' sites that are relevant to my profession (personal branding & networking). But I don't use it for any other purpose. So if your site isn't relevant to my profession, I don't want t…
Seconded. I don't understand OpenID, but it works, and it's not tied to a single (ominous) company.
I somewhat favor the StackOverflow approach, which is "we won't authenticate you, but here's a bunch of services that do", but inevitably I forget which service is actually tied to my account. This is how it's been with my HN account for a while, I loathe deleting the cookie, as I spent 10 minutes figuring out what ClickPass wants.
TripIt has both Facebook and Google Accounts. I'd probably expand to Yahoo! and Live Mail (if MS has anything for that) and leave it there.