>the top OSS projects aren't on GitHub and similar
Hmm?
https://github.com/torvalds/linux
https://github.com/rails/rails
https://github.com/jquery/jquery
Hmm?
https://github.com/torvalds/linux
https://github.com/rails/rails
https://github.com/jquery/jquery
Linus copied the kernel tree to GitHub when the kernel git repo was compromised. It is not the main repo. Of the others, only Rails and JQuery could even be reasonably be called "top" projects (Bootstrap? Seriously, you list a 7 month old project with 2000 commits?) and even those are a stretch. Major projects are well into the hundreds of thousands of commits. It's ludicrous to even muse of Bootstrap being in the same league as e.g. Mozilla, WebKit, et al.
If you define success by the number of people who have actually checked out and built the code, Bootstrap is vastly more successful than Mozilla & WebKit.