It gives them more freedom in package names. It's nice when a package name is the same as the last element of the URL, and that URL's reasonably short. And you repeat the package name whenever you use an identifier from the package, so it's good if it's 1) short, and 2) if possible makes the full identifier read nicely à la counting.Reader.

With their own GitHub username, they can just use whatever word they want for the package name (tool, counting, stack, subset, grace, inject) without risking a collision with other Facebook projects.