The report titled Wave 3 on this site is interesting and includes a lot of global information on use of Web 2.0 tools
http://www.universalmccann.com/
This has some UK information
http://www.nudgelondon.com/blog/2008/11/latest-uk-social-networking-statistics/
and on a completely different note, here is a project using social bookmarking to encourage visitors to engage with museums
Vikki