A lot of people (myself included) either just don't like Facebook, or use it only very sporadically. It's not like you can count on people being logged in all the time -- and on top of this, be willing to share their FB identity with you, just to look at your app.
But the bigger question is: do you really need to have any kind of auth at all?
While you're still in the demo/outreach stage of your app, the important thing is for people to be able to just see the darned thing. Anything like a login is just way too much overhead. It should suffice to create an anonymous session ID, and go from there. And if you find people starting to abuse the site or eat up your resources with their anonymous sessions, well, you can take that as a sign that you're already successful :)
But even if you feel you absolutely must require a login, at least you can have a static "take a tour" section with some canned images and slides. Again, what most people want is just to get an instant idea of what your app is about -- with as little time investment as possible.