Github is not like that. The github engineers quite clearly see github as a product built on git (the technology) rather than a product operating within git (the protocol). They do not improve or contribute to git itself
GitHub has a number of employees that contribute to libgit2 (https://github.com/libgit2/libgit2), and the language bindings for a number of languages. I think that is a serious contribution to the Git ecosystem.
full disclosure: I replaced GitSharp with LibGit2Sharp (the C# bindings to libgit2) in Git-Tfs