Sorry, what I meant by "mainstream" is "have enough of a following". You can say a lot of good things about Bootstrap, and to be fair most of them are probably right, but even when you compare to other jQuery libraries/framework it must have... what, maybe 5% of the users? Also, it's a very young project, it's not an established de facto piece of technology like Django is when it comes to Python web frameworks, Rails to Ruby web frameworks, or Apache is to HTTP servers.
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:
Did I make myself a bit clearer? :)