Social Search & Personalization
Bill Slawski recently highlighted that the original goal of PageRank was to:
…be useful for estimating the amount of attention any document receives on the web since it models human behavior when surfing the web
Google has a personalized home page, recommends gadgets, added many verticals to their organic search results, and biases the search results to your interests. With Google and other engines adding more content types directly to the search results, and adding more ways to search through it, the need for many of the niche communities diminishes. The social communities built on strong brands or bias will want central editors. Communities built on other weaker commonalities will wither.
Many of the smaller social search plays will get buried under their own weight. At this point, their page count will increase faster than their authority does, and as their outbound traffic drops so will their interest and authority. In other words, I think most of the social search plays are at best a fad.