I joined Fotolia in 2006, and am Emerald in rank. My sales graph for FT shows a perfect bell curve from 2006 to now - sales initially grew rapidly, peaking in 2010, and have since declined just as dramatically, currently back to my 2008 levels and still falling. Since the decline started my sales have not improved at all whether I've uploaded regularly or not.
I'm very glad for all the newer contributors who are doing well - just a word of warning that you should be wary of doing too well - if you work like crazy and manage to climb their mountain, instead of supporting your efforts they give you a boot up the backside and push you back down the other side. Oh, and they deny doing it, of course.