There's great power and potential for placemaking with visual social media tools, and I think it would be interesting to create a real-time map from geotagged instagrams. This could be just a general google map, or tied to things like UrbanSpoon, so if you are looking for a spot to eat dinner in your neighborhood, you could pull up the map and look at the latest Instagram photos, which might give you an idea of the food, atmosphere, patrons, etc. in the restaurant.
It would also help create attachment and ownership to community spaces, like public parks, or spaces that aren't in the obvious public eye, like rooftop gardens, or beekeeping colonies. People could look at a map of their neighborhood and get a sense of the happenings in their community, and engage with neighbors to solidify a sense of place.
It seems like a good tool for local government too, who could see how certain spaces are being used, their popularity, and gain insights from those patterns.