What are users interested in and passionate about? Which brands, items (again, brands) or people are talked about the most? And how do people talk about them?
To get a good understanding of that, it is interesting to visit
The Best Stuff in the World.

The popularity of a particular item, person or place is measured in the size of its vignette on the site. You can click on each one and decided whether you agree that 'it's the best' in one or more user-established categories. Today, peace, Futurama, music, The Simpsons, fries, and YouTube are leading based on user votes. Interesting combination...
The funny thing - none of these is a brand per se (with McDonald's, Dr.Pepper and others listed). All have influenced pop culture, they would never show up in the same search result and all but YouTube have been around for years if not centuries.
It reminds me of an advertising microcosm - if enough people know your brand (we'll leave the love or hate out of it for now), its size and visibility grow. The interesting thing is that this microcosm is kept alive and changing by the users, the 'active members', who add new or vote for existing 'stuff'. Depending on how established this site gets, it may actually become a source of insight into the likes and dislikes of teens and twens.
But: why do users participate? All this is done without any incentive for the users. In my opinion it's passion that drives this site - if someone feels strongly about something, be it positive or negative, he is willing to spend the time to express it. I love Milka chocolate and I felt the need to say so.