Stack Overflow wasn't designed to be a place for people to interact with others. It was molded very early on to center around documenting information in an easier to find format. This is a stark contrast with the kinds of forums you describe as part of your early ventures into community management.
And that fact has an extremely important implication on measuring success. It means that participation metrics are not an indicator of success. On the contrary, it means that if new questions dwindle on mature technologies, then that can be an indication that we've succeed at documenting a lot of the information people need, whereas a steady or increasing rate of questions might mean we've failed at doing so. Stack Overflow is essentially designed to work itself out of a job, although it never will because technologies emerge and change constantly.
There is one data point that you didn't mention, and because of Stack Overflow's unique purpose, it's actually the most important one of all: views. Having questions that deliver a great deal of value to future users and can be found easily is vastly more valuable than most of the activities users engage in. Indeed, most of the moderation your veterans undertake is work no one really wants to do. It's not fun or entertaining or even satisfying. When we do it, we do so out of a sense that it's necessary to preserve the thing we do want: a body of useful, clear, searchable questions with accessible answers.
To many of us, it seems that SO the company has forgotten these values. Almost all the data emphasis has been on increasing participation in some form or another, which just creates more of the work we don't want to do and buries the things we actually want to spend time on. A passerby who finds a question mildly interesting or who just wants to answer to get reputation or disagrees with a downvote can skew your vote-base measurements of quality. There is no interest in even analyzing whether questions have any long term value or not. This approach to your community has led to a great deal of lost trust in the company. The continued emphasis on the activities you mention in this talk are only going to make your user base lose more trust.
