I think Git owes a ton of its success to github.com. Without this extremely well-designed, central repository for repositories, the uptake of Git would have been much slower, and faced much more resistance in the wild. Git is a great tool on its own, but having a centralized place for people to learn and use Git has been huge.
In addition to github, you have the switch to git of the linux kernel which really put the project over the other distributed revision control systems such as Darcs and Mercurial.
On the other hand, one could also attribute github's success to git's success. I suppose it's most likely symbiotic: github contributed to git's success and vice versa.
