Zacker.org

1. Moodle: 2,981 deployments / 54% market share
   8,772 X (348 / 1024)
2. Blackboard + WebCT: 2,500 deployments / 45% market share
   3,700 - 1,200 K-12
3. Sakai: 35 deployments / .63% market share

Taken from the Sakai partners page.

Related Posts:
Sakai vs. Moodle
Digging into OpenBRR Rating of Sakai and Moodle