The thing I see about StackOverflow's volunteer moderation is that unlike Facebook, Tweeter, etc.:

  + StackOverflow has clear criteria for expertise.
  + Reputation is based on being helpful to other people.
  + "How do I make baby" is off topic.
  + Content has long term value.

Unlike Facebook, Tweeter, etc. self promotion is bounded because of the purpose of the site. Imagine if you will, a Facebook where reputation is based on the quality of a person's Likes and Friendships as determined by community voting. What gets measured gets optimized. Facebook's user facing metrics don't encourage skills transferable to moderation.

To me, a big difference with StackOverflow is that it is a bounded community from which people do not graduate. Facebook was a bounded community until it expanded beyond people with .edu emails. When Zuckerberg's Harvard cohort graduated and started producing grand-children, the platform had to expand to grand-parents to keep people on the platform. Now that those children need messaging, the platform is expanding to keep people there.

StackOverflow has expanded laterally by adding siloed communities around expertise as the basis for reputation. While its systems have morphed and evolved, there has been a set of underlying ideas that prevent ad hoc change.