Are Pond5 and ShutterStock the Only Games in Town?

These days, it appears that most of my stock footage sales are coming from just two places: Pond5 and ShutterStock. It seems that customers are migrating to these two agencies, leaving the others behind. iStock, for example, has been slowly diminishing in performance since January of 2011, which was my best month there.  In that month, iStock alone accounted for 42% of my sales. By comparison, in January of this year, it has shrunk to 10%. But for every down...