Edward CastronovaInteresting things happening on the interactive social media front. IPO darlings Facebook and Zynga have seen their stock prices falling quite a bit lately. There's more analysis than information, but one thread stuck out at me: The idea that the attention garnered by Zynga and Facebook is not sustainable. For example, Leigh Alexander at Gamasutra carefully unpacks the dyanamics of attention when the portfolio consists of shiny empty games. It works for awhile, but eventually the users become bored at what is after all a very shallow experience. Over at CNN Money, Ben Rooney reports one analyst's conjecture that Facebook...