The Association of Research Libraries has published Social Software in Libraries, SPEC Kit 304. The table of contents and executive summary are freely available.
Here's an excerpt from the press release:
This survey was distributed to the 123 ARL member libraries in February 2008. Sixty-four libraries completed the survey by the March 14 deadline for a response rate of 52%. All but three of the responding libraries report that their library staff uses social software (95%) and one of those three plans to begin using social software in the future.
Survey results indicate that the most broadly adopted social software—chat or instant messaging—was also the earliest implemented social software. While one respondent was using instant messaging for reference and another was using chat for internal communication as early as 1998, the earliest use of this type of social software dates back to 1993.
While chat and instant messaging have been in use for several years, use of other types of social software in libraries is very recent. Beyond isolated cases, a steadily increasing number of ARL member libraries began implementing social software in 2005, with the largest rate of adoption being in 2007.