According to the PPUK blog, the Pirate Party’s Facebook group is bigger than the Labour Party’s Facebook group:
It appears the Pirate Party is more popular than the Labour Party, at least on Facebook. Our group there — at the time of writing — has 6364 supporters, whereas Labour’s has only 6356. And the Pirate Party has only been in existance for a month.
The only UK political party to have more supporters than PPUK on Facebook is the BNP (currently 7383 supporters). This is likely to change, with PPUK overtaking the BNP, not least because there are more filesharers in Britain than fascists.