This graph shows $/month, with a 6th order polynomial trendline for reference. Pretty ugly. But why? Well, not to be paranoid, but they've been putting a big push towards getting contributors to become exclusive, to submit only to them. Part of that push includes things like limiting uploads from non-exclusives, different review process for exclusives, and skewing their search engines to show images from exclusives first, giving them more visability.
I'll continue to work with them, they still perform well for me. And I'd still recommend them to anyone looking at microstock (though not with as much enthusiasm). But the tone of the business relationship has certainly changed.