I wouldn’t characterize stack overflow as a delivery mechanism for ads in the same way that, say, YouTube is. Content creators on stack overflow aren’t there to make money by driving page views. (Rather, the mechanism driving page views is more utility than sensation)
That's a good point. You can't click "monetize" on your answers, but if you could you'd be right in that zone.
I don't know how useful an epicycle in your model this is. Content creators on Facebook aren't there to make money by driving page views either, and yet I assume you wouldn't say that Facebook is "good", by the standard were discussing.
At the same time, SO is hardly optimizing for return visits to SO in the same way that I would assume that Facebook is optimizing their platform for return visits and engagement to Facebook.
On the other hand, every time I go to SO to find a solution to problem ${X}, I find very interesting (to me) questions on the right, enticing me to waste time reading them. They are making an effort to keep people on the Stack Exchange sites.