Not sure I would conclude that debain (or ubuntu to a lesser extent) is "Stagnant" from this data. The count seems to track quite positively w/ each of their release cycles -- every 6 months or ~2ys, respectively, and then settles back down thereafter; however, over all it looks like an upward growth trend (ok, w/ the exception of Ubuntu (fr), but that list has only ~1/10th that of the ubuntu(en)).
I agree that it wasn't easy to make conclusion, especially it compare only one parameter from forums (new registered accounts), and doesn't take mailing lists in account for example, maybe I should leave the conclusion to the reader. The trend for Arch Linux is more obvious I think.
I think this might be a better metric for stability or reliability of the distros, rather than their health: I suspect many people only sign up to these forums when they want to ask for help with a problem. Anecdotally, over many years as an Ubuntu user the only times I've ever visited their forum were when I had problems after an upgrade (and it was usually with WiFi).
I don't have the answer, but when the trend goes to zero, I'll be suspicious about the distro's community, it could be a bug free distribution you're right. On the other side, as a Arch Linux user, I don't think this distro is more buggy that before, and the trend goes clearly up. But you're right, their is not only one reason for the trend's explanation.
Anyone know why there is a steep drop as we move towards the end of 2011?
For some reason, this trend seems to be uniformly noticeable in most distros.
It's only for "per month" graphs, as the last month wasn't complete, it drop at the end. I should have removed the last month for a cleaner graph. "per days" graphs should be fine.