I would have to say Twitter which has become a favourite amongst many.
Marketers love it because of the list building potential and raving fan base that can be amassed.
Famous stars love it for similar reasons on the fan base side. They know a lot of their fans congregate there and fans love it because they have the chance to talk to the people they would never normally be able to.
And ordinary people love it because it's the most interactive personal diary on the planet. People come up with lots of sayings, jokes, truths, trends and even roll playing characters from a sitcom like the king of queens.
For many it's a fun place to be and THAT is why it's the most interactive website on the planet, with hundreds of millions of people.
Having said all that my favourite web 2.0 site is still HUBPAGES :) - just thought I'd mention that lol.