I joined SO when there were are 150K questions. Now there are some 330K questions.
It appears that the number of visits to questions, and the real longevity of an interesting question, appear (IMHO, no empirical data) rapidly dropping off. This seems a like a natural consequence: with floods of new questions coming in, older questions simply disappear in an avalanche.
Disk caches have the same problem in operating systems. If the number of queries that mention cache elements is high enough, the cache elements tend to stay in the cache. If the number of queries for new things exceeds the cache size, the cache gets filled with new items and there's never any cache hits.
So what are the properties of SO as the number of questions goes to infinity? (Or 10 million, that's only 1 question per programmer on the planet and I'm sure we all have at least one)? What properties of the design of SO provides any assurance that it won't die by simply being drowned?
EDIT Aug 7 2011: Now SO has some 1.9 million questions. I go for several days at a time now when no interesting questions come up, in spite of the probably much larger volume of users that exist compared to when I asked the question. Jon Skeet below vaguely answered my original post, but didn't back up with facts. So... I repeat the question... what designed-in properties does SO have to prevent it from getting drowned?