Responding to a LinkedIn Discussion.
When people ask me what makes a MOOC 'massive' I respond in terms of the *capacity* of the MOOC rather than any absolute numbers.
In particular, my focus is on the development of a network structure, as opposed to a group structure, to manage the course. In a network structure there isn't any central focus, for example, a central discussion. Different people discuss different topics in different places (Twitter, Google Groups, Facebook, whatever) as they wish.
Additionally, my understanding is that for the course to be a *course* it has to be more than just a broadcast. Otherwise, 'Adventure Planet' is a MOOC. Or National Geographic Magazine is a MOOC. A course actually requires these interactive and skills development activities, rather than simply consumption of content.
So what is essential to a course being a *massive* open online course, therefore, is that it is not based in a particular environment, isn't characterized by its use of a single platform, but rather by the capacity of the technology supporting the course to enable and engage conversations and activities across multiple platforms.
In the first connectivist MOOC, for example, we have 170 individual blogs created by course participants (in Change11 we had 306 feeds). What made the course a MOOC is that these contributions were comprehended as a *part* of the course, and were all accessible to course participants, either directly, through the newsletter, or through alternative syndication using an OPML list.
Similarly, there wasn't just a Twitter conversation, or Second Life event, that happened coincidentally with the course, but rather, these events outside the 'main platform' were construed as part of the course, and comprehended in the course description, relevant links, newsletters, etc.
The big danger, to my mind, in a large online course is that through strong group-formation activities, it can become a small online course. This happens when a central clique or insider group is formed, or where you have inner circles and outer circles. The inner circle, for example, might expect and demand preferential access to and individual attention from the course facilitators.
When this happens the dynamics of the course change - for example (and not these are my observations and opinions, not hard established fact) the primary value becomes cohesion and agreement, rather than diversity and distinct perspectives.
When the course functions as a small group, there is an expectation that everyone will agree on the course content, objectives, and domain of discussion. But, in fact, to be a massive course, it must needs respect a wide variety of individual objectives, perspectives on course materials, and opinions about relevant topics of discussion (not to mention technological platform and language of 'instruction').
Consequently, when I have been asked in the past what number a course needs to attain in order to be considered 'massive', after providing the caveat just given above, I provide the figure of 150, Dunbar's Number, as the cut-off line.
Now to be clear, this would refer to *active* participants, and not merely the number of people who signed up. Thus for example the course that has 170 active blogs *does* qualify, while CFHE, which had 83 blogs, is on the cusp (it would need another 70 people active on other platforms, such as Twitter or Google Groups).
Why Dunbar's number? The reason is that it represents the maximum (theoretical) number of people a person can reasonably interact with. How many blogs can a person read, follow and respond to? Maybe around 150, if Dunbar is correct. Which means that if we have 170 blogs, then the blogs don't constitute a 'core' - people begin to be selective about which blogs they're reading, and different (and interacting) subcommunities can form.
I sketch the difference between 'groups and networks' in this diagram - http://www.flickr.com/photos/stephen_downes/252157734/