I think it would be fantastic if we could do a statistical analysis of whatever usage we can get our hands on, including StackExchange and GitHub. Markdown has a broader use than just “social” sites, so I think it’d be great if we can expand our analysis beyond the “social” aspect.
For instance, Markdown is used quite a bit on blogs (which aren’t entirely social), and we could look at to Wordpress.com for usage data. Wordpress.com has has markdown support, an implementation of MarkdownExtra which famously has support for tables and footnotes, and looks to be under somewhat active development. Apparently 43.7 million posts are made on Wordpress.com monthly, and while I don’t know how many use the “extra” features, we could ask. A search of Wordpress.org’s plugin repository shows that plugins supporting Markdown have hundreds of thousands of downloads. Again, no way to know how those downloads are used, but we could do some rough estimates.
As an aside: I’m not trying to “prove” the need for tables or footnotes (although I personally find them valuable), more I am hoping that we can base our implementation on common usage in the real-world, and use data to make our decisions about what to include. As you note, code blocks were not in the original implementation but are incredibly useful. Just because something wasn’t imagined by Markdown when it was created, doesn’t mean that a great solution hasn’t been found for many users.
EDIT: MarkdownExtra is discussed more here.