Personally, I have signed up for maybe 50 MOOCs, but only completed 2, and really engaged in any level with maybe 3-5. But this was intentional. Signing up lets me:
1) Access the videos/materials for all time (even when the course is no longer open).
2) Try out different courses / professors
3) Learn the basics (i.e., watch only week 1 out of 6); or perhaps watch all videos but ignore the testing materials. After all, if there is no real credential involved, I might only care about gaining an understanding of the subject, rather than prove I understand it through testing.
I am a full time student (not in CS), and in addition to my real life course load I am currently taking Networking, Cryptography, and Algorithms. There is no way I will have the time to complete more than 1, or MAYBE 2, of the 3.
In other words, I view my low completion rate as a huge success. Coursera seems to have realized this fact. They have just started doing pre-course surveys that ask you how many videos you intend to watch (none, 25%, 50%, all), and they ask how much of the homework you intend to do.
Of course, offering degrees or college credit, especially in exchange for cash, will obviously increase completion rates, but mainly by dissuading people like me from signing up.