The fact that so many games have Facebook only login is a sign that when done right, with a demographic of players that accept it (almost all, the SF Bay Area is way more anti facebook-login and sharing than most of the rest of the world), the players you lose by having only Facebook login are made up for by the viral spreading you get from Facebook users.
The caveat of course being that you have to design, from the ground up, for the game to spread through Facebook. If you don't, it's not worth it. But if you don't, you need some other strategy to make the game spread, and there are very very few unfortunately.
Finding the right demographic would be great, I think at this point we're still trying to figure out what to do with the game play to make it fun, clean up bugs, tutorials etc.
We're also working on the android version, mobile we think will be a huge driver for this game (in our mind).
Really do appreciate the feedback, and we'd love to hear more from you
How could having the option of registering sans Facebook be a mistake? In what way could broadening your target audience be a poor choice?
Hinging your cart entirely like that (becoming a sharecropper that is completely dependent on the goodwill and cooperation of a much bigger partner) is seldom a responsible business strategy.
But I also have some specific reasoning that leads me to believe it's not a poor choice to provide another way to sign in. Signing in with facebook is extremely easy compared to normal signup paths. What this means is that most people who trust you with facebook will still sign in via facebook. And I don't think there's much of an argument that people who don't trust you with facebook will be signing in with facebook if it's the only option.
You can always allow them to link with facebook later, after they know what the game is, and after you've explained to them why linking will be valuable to them.
You're assuming that more options always increases value, but this is not the case. See Apple and, let's say, the Samsung Jitterbug for counter-examples.
However the claim I was debating was that a second sign-in method would be bad because people would exercise it. The argument was basically that it's bad because it's increasing value for users in a way that decreases value for you. And I don't think adding a second sign-in method to a page that currently only has one button really causes a difficult or confusing decision or creates a cluttered UI.
I suppose I still do see your point, though, since I personally wouldn't go much beyond that -- "sign in with facebook or twitter or google or browserid or github or create an account or or or" sounds like a miserable experience.
Presumably, because there exists a population of people who will log in via FB if that's the only option, but will log in using another option instead if it's available.
Depending on the size of this population and the added value of having plays log in via FB instead of another method, it might outweigh the benefit of capturing those players who will never log in via FB but may play if another log in method is available.