Yeah, sadly this is a nonstarter for me due to this. Perhaps somebody will expand the idea outside of the Facebook ecosystem, but I'm not about to sign up for Facebook just to give this a try.
I tried again with a neighboring zip, but this time I hesitated for a moment to make sure I entered it right, and the search box did a weird thing where it popped down an expanded city name along with the zip code. I clicked this and THEN smacked enter, and I got a search results page!
Of course there are no results in my area (and no way to tell how large a radius it considers "my area"; should I try again with a zip a few miles away? a few dozen miles away?), but now I know there's a secret-knock to get into the site.