I don't necessarily think this is a bad thing.
For Stanford's database class that just finished we had 64 thousand register, but only 20,000 actually do some work, and then close to 5k actually take the final or get a statement of accomplishment.
My blog response: http://sef.kloninger.com/2013/03/online-ed-retention/
I'm the engineering manager on Class2Go, Stanford's open-source MOOC platform. Check it out: http://class2go.stanford.edu/