Libraries are struggling to show their value to the community. One way is to leverage check-ins from location based services to show that people are using the library a lot. The nice thing about using such metrics to measure foot traffic is that it's available for all venues.
I found that for example, in my university, the library is the most check-in place on campus by far, even though no attempt was made to claim or promote it. That said FourSquare and similar services are still primarily used by early adopters so this is just an early indication, but this will change with Facebook places (available in US only as of writing).