Flat works at the small size of GitHub and Valve. But I can't think of a single large company that is flat. Where is the evidence that a flat structure even works at the size of these companies?

Just so the comparison is clear:

Microsoft: 94000 employees [0]

GitHub: 140 [1]

In other words, Microsoft is 670x larger. Or GitHub is 0.15% of MS's size.

[0] http://www.microsoft.com/en-us/news/inside_ms.aspx

[1] http://bits.blogs.nytimes.com/2012/12/28/github-has-big-drea...

Gore & Associates is the major counterexample-flat structure, 9,000 employees: http://en.wikipedia.org/wiki/Gore_and_Associates

You need coordination and guidance at 94,000 people. You can't have it be totally "flat"; I agree. You need some administration and some conceptual structure and hierarchy is the way humans think.

You don't, however, need managerial extortionists who won't let people seek independent credibility, choose their own projects and teams, or collaborate extra-hierarchically.