Oh boy. I don't really care for many of these social media sites, especially Facebook. But the Foursquare one looked interesting so I decided to investigate it.
Overall I liked the tips portion of this the best. Adjusting the recliner in a specific place during a certain time of day for maximum sun exposure! Especially helpful during this rough winter we have had. I think I would like to be reincarnated as a turtle so I could sit on a log all summer long and sun myself...
This also appealed to me on a personal level. Ever since my failed career as a landscape architect I like pointing out what I see as "failed" levels of design of particular buildings or landscapes. Adding info about how they could have done a better job of handling the sidewalks, lighting, or orientation to the sun might be able to spark conservation about particular sites of interest. Life without conversation or debate can be pretty boring, right?
As far as applications for libraries, adding where in your library the most comfortable seat is, or where the best newspapers are is something that could draw potential "on the fence" patrons inside. Those that might not ordinarily want to visit your library could have their interest piqued if somebody wrote an amusing tip on Foursquare.