I like the idea but when I first read your description and went to the site, I expected the equivalent of a cork board in an office lunchroom, which it does do with "gatherings", but the rest feels like Foursquare tips or Yelp's reviews. Basically, I thought it was something your going to read while you wait for the coffee to brew that has targeted information about the office "community" or "neighborhood". "I'm selling my car" or "Employee meeting on Friday the 13th". I'm using the office as an analogy like you used the flyer on the tree. But the true question here is why would I go to your site to find this information? When I walk down the street I have no choice but to run into the tree with the flyer. Then I might tell my neighbor that Cindy lost her kitten and that she should look out for it. Maybe it's easier to start spreading the local info on your site via word of mouth (existing social networks) so that people hear about your tree (your site) and at some point they'll get used to looking for the flyer on the tree.

Ok, sorry for the analogies and generalities.

I think it's going to be a challenge to get people to use it since most people feel saturated by status updates / check-ins / review apps. I'd suggest trying to leverage other social network api's in order to automatically group people by location and also facilitate posting to your site. Maybe create a light weight Facebook app that looks like a tree with flyers on it?

Hope this helps and good luck.