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.