'Public library' as defined by the IMLS is found below.
2012 update posted on 2/4/2015.
According to Appendix A of the 2012 Public Libraries in the United States Survey -- at least as I read it -- the changes in the numbers for 2012 are due primarily to response rate (97.4%) and shifting among population categories.
The reason for a drop in the number of libraries in 2010?
It had much more to do with conforming to the FSCS definition of a public library provided below than the actual closing of libraries.