The magic lies in the two-sided coin of promotion vs. spam filtering.

The web started off as a pretty peer to peer system, but almost immediately people built directories and link farms as means to find things. You can make a system as distributed as you want, but that only works for content which people know to find. Which is great for piracy, as e.g. movies and TV shows are advertised everywhere else and can be found by title.

For social media, the recommendation engine is a critical part of the appeal to users.