I found the Link
about the survey. A quote from it:
"Nearly 60 percent of the 865 microstock photographers participating in the survey reported earning less than $1,000 per year."
I found this quote more disturbing though:
"Second, she is exclusive to iStock; she earns 40 percent of individual image sales, double that of nonexclusive photographers."
I thought galleries were greedy but this takes the cake.
PS The article links to a good blog: here Microstock Diaries