Without having seen the app myself, could it be the case that it's designed in such a way to "strongly encourage" facebook signup? One of the design tradeoffs you or your designer may have made, consciously or unconsciously, was that some percentage of people might misinterpret your design as facebook-only unless there are very obvious signs to the contrary.

Of course, I haven't used the app myself, so I can't take one side or another on this one. Come out with an Android version and we'll talk. :) Either way, it looks like a cool product, so, kudos.