well... I spend the most time on Google as I am obsessed with the positions of various websites I work with. Does that count? Plus mumsnet, obviously.
I also like sites with statistics where you can just flick from one thing to another. The WHO (I used Google last yime - www.who.com I think) is fascinating - you can compare the suicide / road accident stats in many countries... (how sad am I? )
But for chat etc. mumsnet beats all the other sites I've tried - either they don't have enough users, or they have too many and you find yourself writing a brilliant earth shattering argument, only to find the discussion has moved on by the time you click "send" (guardianunlimited is like this). Or they've been designed by idiots.