Both Shutterstock and Dreamstime have announced reaching image upload milestones. Shutterstock have added 2 million images to their collection in the past 10 months bringing their total image count to 5 million. Dreamstime  announced that they had accepted their 4 millionth image, taking on 2 million new images in less than 12 months, dreamstime also highlight that they now have 1 million registered users, I just wish they were all actively buying!
Clearly the upload rate is accelerating (dramatically), shutterstocks' site proudly boasts adding 70,000 images per week, at that rate, if constant, they will add more than 3.6 million photos and illustrations in the next year; I suspect it will be more.
"We are extremely proud to reach the 5 million image mark, especially while maintaining our stringent standards for accepting new images. We are certainly grateful to our global base of more than 120,000 talented submitters who provide our subscribers with a steady pipeline of great creative content." -- shutterstock
Dreamstime reports having "nearly 50,000 contributing photographers from novice to the professional."