GitHub is a company, not a technology. "Embrace, extend, and extinguish" doesn't apply. Unless of course you're worried about them doing that to git itself, not just GitHub.
1. Embrace Git by buying Github
2. Extend Git, fork git to add Github specific features (Basically integrate https://hub.github.com/ directly into git)
3. Extinguish, by gradually making Github incompatible with the original git. Then stop development on the forked git and push everyone to a proprietary product.
They are not going to slowly change git to the point where no one else can use it. Git is opened sourced, by owning GitHub they still can't control Git anyway.