Is the perception that we will be nefariously ingesting your private data? Or that FB auth is just a trend? Or just a mistaken assumption that all users actually have a Facebook account?
I also think it is a bad idea to outsource your authentication mechanism to a single private entity. What happens if your user deletes their account? What if Facebook thinks your site exhibits suspicious behavior and decides to not send along its users' authentication? Probably won't happen but if it does it could be a world of hurt, much like PayPal.
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 to connect via Facebook, and if that's the only login option, you've just lost me.
Every site should have OpenID and/or its own user accounts, so as not to lose users who for one reason or another don't want ot use FB.
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.
And guess what, a facebook account is also an open id.
Happily, google is an openid endpoint, and basically everyone has a google account for gmail or something.