If it wasn't immediately obvious why Comcast forked over a reported $150 million for Plaxo, a social networking site, it may become clearer later this month, when Plaxo officially launches Fan Pages for FanCast, Comcast's online video site. The social features available in beta now let users join fan groups of various TV shows, where they can share episodes and discuss plot developments to their heart's content with fellow obsessives.