A recent article, Students are Streaming to State Virtual Schools shows that online school enrollment is increasing dramatically because of state initiated virtual schools. Utah leads the list with about 49,000 students, 
followed by Florida with 31,000. Adding other states and programs by universities brings the total to about 140,000 students. Most of these programs are created by legislatures to supplement course offerings in regular schools. A more detailed report is Keeping Pace with K-12 Online Learning: A Review of State-Level Policy and Practice.
In addition, virtual school enrollment is also exploding most often in virtual charter schools sometimes as programs by school districts or for-profit organizations according to The North American Council for Online Learning.
Also, some 2.5 million students in higher education take a course or entire programs online.