A while back, we had this discussion in which we tried to measure quantitatively whether the site was gaining or losing non-novice users. Since then, I've had the distinct subjective impression that the quality of discussion on the site has gone down markedly. I went through some of my own questions to which I'd accepted an answer, figuring that the people who wrote those answers would be the kind of competent and knowledgeable users who would make my own experience here better. We do seem to have lost some, including Cristi Stoica (inactive) and Luboš Motl (very low activity).
I tried using some database queries to test this:
This is Manishearth's query that he formed as part of the previous discussion. It attempts to measure the number of users who are not novices, based on whether they post multiple times in a month in certain tagged areas. Re-running the query now shows that the level of such users seems to have been stagnant for about the last year or more.
This is the number of users posting in homework. It shows strong growth over the last year.
This is the total number of active users. It also shows a strong, steady trend of growth.
The reason I'm using the term "critical mass" in the title of this question is that the critical mass of, say, uranium-235 depends on concentration. If you take a given number of 235U atoms and dilute them, you can make a critical mass into something that's not a critical mass.
Subjectively, I'm finding that I've been spending my time on the site in unenjoyable activities like arguing with angry and belligerent beginners about friction and Newton's third law. I've decided to become inactive on the site.