While it's not nearly as old as those projects, it may be used by the same order of magnitude of people. Both bootstrap and jQuery itself are officially hosted on Github:

    jQuery    - 16,896 watching, 2,314 forks
    bootstrap - 38,026 watching, 8,836 forks

There are a lot of ways to interpret those numbers, but I think it at least shows that there is serious, broad interest in bootstrap.