Retention rates are low in everything, not just online classes. Ultimately, its how the student reacts to the plateau. Anyone in any area of teaching, I'd highly recommend reading "Mastery" by George Leonard. In the book, he talks about 3 of learning types that can stifle progress: the Dabbler, the Obsessive, and the Hacker (not to be confused with the IT world's rough definition).

* The Dabbler will pick up tons of things, "overjoyed" about the newness that they are learning. Eventually, they hit their first plateau and the 'newness' becomes boring and they'll seek out another activity to experience the same 'newness' high. Maybe they'll come back to it later and see if they can progress again.

* The Obsessive will seek out perfection on this new activity, pouring over every YouTube video or article to figure out how to be better. When they plateau, they'll try harder; hoping for the same results. If the return isn't there, the Obsessive will look for another activity.

* The Hacker will actually ride out the plateau potentially indefinitely. In some cases, this will create progress eventually, but in others, it can become the cap of their learning capability (hence, 'a hack'). They just don't put the extra effort needed to progress further.

While all these types are negative, two-thirds drop the activity for something new, something they might be a natural at. From a business perspective, a monthly subscription favors Hacker-type customers; where a 'per course' pricing scheme favors Dabblers.

Steering back to online courses, I think the issue isn't 100% the site's fault; but that certain students will join for the excitement, but fan out eventually (Leonard goes on later to discuss how to better approach each of these types as the Teacher). I've signed up to all of the sites in the article, along with Udacity (where I'm currently taking the Web Dev course). For Udacity and edX, I've completed the courses I've signed up for; however at Duolingo, I stopped after the Basics 1 in Spanish. I want to learn Spanish, but the drive and time isn't there for me.

Jumping back to 'Mastery', Leonard talks about 5 keys that can help promote mastery and can be used to retain students: Instruction, Practice, Surrender, Intentionality, and The Edge.

* Instruction is obvious: have great teachers(not just through credentials, but are generally good AT teaching), something these sites offer tenfold.

* Practice is where I felt edX's course fell through. It was a lot of lecture, but the Finger Exercises were more the tradition 'regurgitation' of knowledge from traditional classes. Duolingo and Udacity (at least the Web course) seem to have this concept taken care of. Duolingo offers different ways to practice and at least with Web Development, you can to see the fruits of your labor with each class.

* Surrender is more for the student than the instructor, but talks about accepting you aren't a Master as something and so sometimes you need to take two steps back to take one step forward (in Aikido, we refer to this a 'Kihon Waza', or practicing the basics). So many people have the mindset of "If you're not doing it this way, you're doing it the wrong way", that this can stifle learning. As an instructor, thinking up new and creative ways to practice the basics can help alleviate this.

* Intentionality talks about visualizing the action, and while focused more on physical activities, could be utilized online. Give the students a view at the end results/expected return on their time. Sure, its dangling the proverbial 'carrot on a stick' in front of them, but it can help them sees where the fruits of their labor. If you can mix this with some of the Practice key, you could get better results (IMO).

* The Edge tip-toes the line between endless practice and pushing the envelope (though Leonard says it isn't). For a new student, this can be a little harder, considering they are still learning; but creation of 'outside the box' milestone/projects could benefit from this. The student's amassed this bit of knowledge; make them use it in a less than cookie cutter way. This would be a great undertaking by online courses, due to how do you 'personally' grade someone's submission, especially overtime? Maybe setting it up in a 'there is no right/wrong answer' way, namely because if a student takes a risk, they shouldn't be penalized for it.

Like I said, I'd recommend the book. I teach community college currently, and while most this is anecdotal opinion, I feel like it's helped me maintain engaged students.