Git was popular before GitHub was a thing. Git was taking over the open source world back when everybody was on SVN, and people were just hosting their Git repos themselves. GitHub made Git ubiquitous for all types of developers and accelerated that adoption further, giving us the near-monopoly we see today.
A better statement is that Github rather killed the momentum of Mercurial, which was also picking up steam in that era (Python, OpenJDK, and Mozilla all chose Mercurial over git).
