This is a pretty cool concept. It obviously has a lot of overlap with Meetup (which I've used for years) though I like the fact that it is more immediate. Meetup tends to involve RSVP'ing for an event upfront and many times events are closed days in advance. As a business owner myself, sometimes I just don't know when I'll have time off and like the spontaneity that something like Four&Me could offer. I think you need to start with a specific region though. It could be quite discouraging for users to login somewhere (like London) and see nothing happening around them. It makes it more likely to uninstall the app and not even consider using it again... Not quite sure how you could go about doing this.

To address some of the criticism surrounding facebook logins - yes, this may be a put-off for some people, but then it has also worked remarkably well for a lot of apps, and for most non-tech-savvy users, it changes the registration process from filling in multiple fields, to a couple of clicks - I personally think there is an appeal to that, particularly for a new product. Plus, you're not asking for too much from users when they connect through FB (though perhaps you could cut this down even further - do you really need my interests? Or could this be another step where I could just click and type a few myself?).

Good luck with the app! Hope to see it grow!