From the little explanation offered, this sounds like a great idea. I'm often going to the cinema by myself because none of my close friends are into the same movies as me. Plus there could be room to expand into different niches with it, like dating.
I think having the Facebook sign in is fair in this case.
On the downside, the appearance of the site looks pretty poor, a classic case of bootstrapitis.
This is something http://www.swarmly.co/ should get into. It doesn't have much users but imagine you announce that you're going to a park. Only those that arrived plus or minus 5 minutes, that may or may not have the same interest, are shown; it shows up with a pop up. Great for central park and people with a certain type of dog you want to see.
I think having the Facebook sign in is fair in this case.
On the downside, the appearance of the site looks pretty poor, a classic case of bootstrapitis.