Sloan-C - K-12 Online Learning Survey
Meta-Analysis Shows Online Learning Better, Blended Even Better
According to a meta-analysis by the US Department of Education, face-to-face courses are less effective than online and blended learning. They caution against viewing this as simply a matter of the medium though. It's the changes in what online and blended learning allow (like opportunities for collaboration) that are likely making the difference.
Direct link to PDF: http://www2.ed.gov/rschstat/eval/tech/evidence-based-practices/finalreport.pdf
The meta-analysis findings do not support simply putting an existing course online, but they do support redesigning instruction to incorporate additional learning opportunities online.
Will at Work Learning: Major Research Review on eLearning Effectiveness
Will Thalheimer reviews the research on workplace learning to answer the question, "Does eLearning work?" He concludes that elearning does work, and blended learning works even better. The difference is based mostly on the instructional methods used, not the media.