CILIP, LIBER, New Academic Librarians, YEP! and Irish Public Libraries and sometimes an interesting discussion crops up on one or other of these, but nothing worth writing home about.
Two new networks aimed at UK librarians have been set up, both of which I think will be more useful to me professionally than LinkedIn or Facebook. The LIS New Professionals Network was set up to support, well, new professionals in the library and information sector (what gave it away?). There are already some interesting discussions, useful resources and a blog to follow on this site.
The second network (as mentioned previously) is the Librarians as Teachers Network, set up literally this week by Johanna Anderson to support librarians in their teaching role. This site is very much in development, so come along and help us to decide what should be on it!
Edited to add: I've just heard of academia.edu via Magister et Mater's blog and am off to explore. It seems to be closely linked to Facebook though...