Kang Dels In the current stage of the product we didn’t need that (and we took in account that some people that don’t have Facebook might not use our app). However, the conversion rates were good enough so we didn’t need to worry about that at the time.
Anyway, if I had to design another screen with two sign up option (e.g. Facebook and Twitter) I would try as first attempt to make a “two way” slider — ^slide right^ takes you to Facebook signup and ^slide left^ takes to Twitter/other signup.
I’m glad to hear that you found it interesting!