These statistics are based off of alexa.com.
Rank - Site
323 Istockphoto
1341 Shutterstock
1978 Fotolia
2091 Dreamstime
3708 StockXpert
6912 Bigstockphoto
9895 123RF
22746 Luckyoliver
23705 Crestock
31794 Featurepics
59967 Canstockphoto
74848 Albumo
110481 Moodboard
120747 Vectorstock
171012 Snapvillage
215784 Regimages
264641 Zymmetrical
278655 Microstockphoto
554574 Scanstockphoto
560346 Fotomind
562888 Imagevortex
803702 Usphotostock
821045 Stockphotomedia
1041191 Photola
2879694 Paxxion
5403136 Thephotostorage
To compare:
107790 Microstockgroup