Perhaps the reason Mono has no traffic on their mailing list is because Mono as a piece of software kind of sucks, at least in my opinion. Then again my perception might just be colored by my experience with using OpenEmbedded which I've come to dislike greatly and coincidentally also uses Mono.
I think more charitably it's because mono is in some senses a write-only project. It's a collection of people who are essentially writing software to an external spec. Except for design details at the very core (probably handled via private mail between the half dozen people involved), there really isn't any need for elaborate collaboration on development details. And futures issues (the "how do I do this?" kind of discussion) are all directed at microsoft anyway, not mono.
I guess I'd have expected it to be a little sterile. Certainly it isn't something I'm dying to work on.
It seems I've miss spoke. I was actually thinking of Monotone not Mono ( http://monotone.ca/ ) I was just confused because I assumed the author was comparing version control systems. But they were actually just comparing open source projects in general. I have no opinion on Mono but Monotone sucks.