Keep in mind that Postgresql wasn't as in vogue when Github was first built, and most or all of the NoSQL solutions you're thinking of didn't even exist. Up until 2 years ago, they were running Rails 2.3, and then is when they moved to Rails 3 (Rails 4 had been out a year at that point). I am very assured that Github is very careful about their technology choices and not quick to chase trends, given the importance of their application.