I'm sorry, but its just not worth expending the extra effort to get you signed up to my service. I can get millions of people before it even starts becoming an issue.
No thanks, and the fact that I'm not the only one means that if you're using FB to authenticate as a startup (IE need to get as many PURCHASING customers as possible), you're doing it wrong.
If I like your service enough, I will make sure to tell people about it. You do not need to try to do that job for me. That's insulting.
Edit: Please note that I realise that you may be an exception if it is something whose market IS a Facebook user (apps, etc), and I am simply addressing the issue as a whole as I have seen completely disparate sites that require a FB login for no reason that I can fathom.
1) It could, potentially, provide some value to you as a user. Quora gets to suggest people to follow for you based on your Facebook friends, so your experience on there is seeded with relevant information.
2) Nothing you do on the site you signed in with is published to your Facebook profile, without your explicit consent on a dialog box.
The trouble with this statement is that some users (myself included) don't believe (a) this is true even when they say it's true, or (b) that if it's true today it will still be true tomorrow.
Such paranoid users are worried that FB will make a privacy policy change that turns the privacy off "as a benefit to users," and the opt-out checkbox will be buried seven links deep. I try to use FB's controls to make my FB stuff fairly private, but I still operate on the assumption that one day FB will break my assumptions about what is or isn't shared.
The recent "Places" launch confirmed it for me. If I hadn't read someone else's blog post, I wouldn't have known that simply refusing to opt into places wasn't enough, I also had to explicitly block friends from checking me into locations.
Which brings us full-circle back tot the point of the post:
When a third-party application uses FB as its authentication mechanism, it gives the appearance of asking its users to trust FB with everything they do on that application.
So yeah, I don't put anything on FB that I can't handle becoming public some day. That doesn't mean I want it to be public, but I wouldn't knowingly put something private on there.
And that extends to third-party apps using FB for anything at all. I can't ever imagine using a linked-in kind of application that uses FB authentication. I'm not going to put certain business contacts and my business relationship with them where FB might be able to scrape the data.
I'm not dating, but if I did I wouldn't use a service that used FB for authentication. Or a personal money management application.
And my message to third party apps using FB for authentication is to take this into account. I won't say "don't," you know your market, maybe they don't care. But at least have your eyes open to people who might think twice if whatever you're managing for them might be sensitive.
Were I a startup founder, I would make it my goal to ensure that EVERY. single. potential customer can use my site, within my capabilities.
(edit: I don't know why you're getting downvoted; you stated your philosophy as part of the discussion, which I don't think is a good reason to get downvoted.)
Users will only give you one chance, if they see a reason not to come back then they won't come back, regardless of later changes you make.
When you're building a startup, in my experience you want to advance on the narrowest front you can, as you build out the feature set and reach the market. Don't provide 5 ways to do X if you can just provide 4. Or just provide 1. Etc. There'll always be room in the future to iterate and add support for additional use cases, additional integration points, and additional polish. But you generally want to get to market fast, get real users, real customers, validate your market assumptions and business model assumptions, and slow or stop your burn rate before your runway runs out.
Saying NO to some things frees up additional time/money to say YES to others. So you should prioritize.
Usually your addressable market will be something like: "English speakers over the age 22 with household income of at least $30,000 per year."
And then suddenly more people are on Facebook than not.
In answer to my own unasked question, yes, there may be logins available on bugmenot (http://www.bugmenot.com/view/facebook.bug) although when you search for 'facebook' it says it says that it has been 'barred'.