I don't think this one-website notion would work in practice since most (well, nearly all that I know of) websites have outbound links and refer to offsite content.
This is a tough question because there are quite a number of websites I frequent and they are all useful, important, or entertaining in different ways. In terms of time spent on an activity, I would have to go with geocaching.com but then I'd have to ask why not Where's George, which I've been participating in for a longer time? Perhaps a portal site (for example Yahoo, if you include all their various services like Flickr and Delicious, or Google, if you include services like Reader, Gmail, Groups, and Youtube) would be a more practical answer to this question.
That said, I'd pick Friendfeed because people I follow there are always sharing and discussing interesting content. Although most Friendfeed entries point to offsite content, there's a good deal of onsite content because you can microblog and photoblog there too.