Humour on the Web
More than anywhere else, humour on the Web is constructed socially: an initial utterance is responded to, and so on in a chain that builds up the joke.
Witness Slashdot for some hilarious commenting activity where the initial article, the seed information, only serves to elicit the responses from the community. Responses are typically orthogonal to the original article and experienced commenters answer each other, building up the humorous threads of response. (It's a shame we can't isolate these threads on the discussion page - or can we?)