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> Github and Reddit (both the largest users of Markdown that I'm familiar with).

StackOverflow? (who're also on board via Atwood)

Atwood hasn't been associated with StackOverflow for over 2 years: http://blog.codinghorror.com/farewell-stack-exchange/

That said I'm sure he has valuable experience with Markdown and is almost certainly using it again in his latest venture.

Ben is listed on the site and works at Stack Exchange right now.

And Atlassian, sort of. When they feel like it. BB uses Markdown, but most of their other stuff is something else entirely.