According to a Nielsen's Online report the top ranking social media sites in the UK are:
(YouTube>Wikipedia>)Facebook>Blogger>MySpace>Bebo> Slide>Yahoo!Answers>LiveSpaces>TripAdvisor
Top Social Media Sites In The UK - Jan 2008 | AccuraCast Search Daily News
If I were you I'd stick to Facebook, MySpace, Blogger, Digg, StumbleUpon and Del.icio.us
Anything else depends on the audience of your blog