UPDATE: IMLS Public Libraries in the US Survey (FY 2013) Number of Libraries (Administrative Units)

Source:  Institute of Museum and Library Services

'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. 

Source:  Public Libraries in the United States Survey (2011)

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. 

Although this situation occurred in 2011, it provides an example of a library that no longer fits the above definition.