Just my personal opinion, but I would suggest having something like: "Don't want to sign in with facebook? We understand, here's why we're requiring it while we

Just my personal opinion, but I would suggest having something like: "Don't want to sign in with facebook? We understand, here's why we're requiring it while we build: {...}, enter your e-mail here [email input box] and we'll notify you as soon as other login options become available". This would serve to explain why you are requiring it (which might be enough to persuade some folks) and would allow those who are interested but not enough to link their FB to have a way to keep in the loop. As far as how long you can get away without having your own login system, it really would depend who your target market is - and what kind of community you are trying to build. I would recommend keeping track of # of sign-ups per week / month, and adjust the priority of the "create login system" task based on how that number changes.