I'm new to microstock (about 6 weeks) and my sales (order--from best to worst) run close to what is shown on the Poll Results here (SS, FT, IS, DT, 123RF, Alamy).   But what really accounts for the differences?  I understand that different clients choose to work with different sites, but is there a type of photo or subject that certain sites specialize in or type of client they cater to?

The other big thing is that some agencies have made themselves easy to work with and thus appealing to buyers. Their site is useful, attractive and works (fast) almost all the time; the search is good in returning them relevant results; prices and packages of credits or subscriptions are appealing.

Once upon a time, iStock did a better job at the above than anyone else; right now Shutterstock is at the top of the heap.

Inmagine is the parent company of 123rf (and they're based in Malaysia I think https://www.linkedin.com/pub/andy-sitt/24/60a/7b2) and they have distribution partners everywhere - some agencies have extensive networks of other outlets that your work may be sold through (although I think that business model is going to disappear in time).

Fotolia was the first microstock agency to offer local language and currency support (everyone has since followed suit, with varying success and different approaches to localization). It has left them as the big dog in Germany.

For a while, illustrators did much better at Canstock than photographers (for whom the agency barely sells anything any more). Pond5 does well for video but not much for photos and illustrations.

Bottom line is that unless you have very specialized content, the income will come from where the bulk of the buyers are.