I think, monthly % increase (and BME) provides trends and great regret that they are true. They reflect SEARCH buyers. It is true that influenced the various microstock sites in their algorithms (sample bestmatch) and from the dynamics of submission / acceptance (SS weekly, from 2010y - over 120000images -> 2011y - 70000images ).
A size and quality of the portfolio does not matter where the trends - they concern only the specific contributor.
Below are weekly DL cycles of two, one (red line) with 750, second (blue line) with 8500 photos - deviation is repeated 1:1 - in Shutterstock
This for period from Jan.2011 - Fev.2011
Black lines is trends.
I teach (of course analyzing trends) workshop for microstock photography
My Mart 2011:
SS, +13% BME (+12% 2010)
VEER +626% BME
IS +76%
Depositphotos BME
/Sorry for bad english, thanks Google translate/