“SO has probably added billions of dollars of value to the world in increased programmer productivity.”

No: the voluntary contributions of many skilled programmers, mediated by a website, has increased programmer productivity. Stack Overflow has captured and exploited some of the value created, and the contributors have not been paid.

Skill needed to create SO / Expertise embodied in content provided for free to SO = approx zero.

Running a big site like SO is hard, but it’s trivial compared to the expertise required to write the content on SO from scratch.