NYC might not have had any 'runaway' successes recently, but it's had some rather respectable advertising-related successes in the last few years - for example, Right Media, TACODA, and Quigo.  The employees at these companies have largely gone on to take more-responsible positions at a variety of other advertising-related startups, many of which are doing rather well.  When it comes to online advertising, at least, the talent pool over here is getting pretty deep.