If it didn't have to make any money and was guaranteed to have visitors, I'd make Reddit with autogenerated subreddits.

At the start there's one community everyone is a part of. Every week, the community is split into children based on voting patterns, grouping people who are more similar. Communities can vote on a name, and anyone can see any node in the tree.

There would have to be periodic resets, but I think it would be a fun social experiment.