It's estimated that
jQuery is employed by over half of all "major" websites. "Major" means highly-trafficked-sites such as NBC, Amazon, Twitter and ESPN. Close to one-in-four of these sites use Google's hosted jQuery library files (a.k.a. Google's "Content Delivery Network" or "CDN") rather than serving local copies of files. There are some great reasons to deliver jQuery via a CDN -- namely a better end-user experience and less server and network loads thanks to CDN content originating from localized source servers and end-user web browser caching of commonly-served files. Employing a CDN, however, introduces an external dependency that can cause sites to seriously malfunction if the CDN becomes unavailable.
The easy solution is to
dual-source those critical jQuery files: attempt to load from a CDN and, if that fails, load from a local source instead. The code for this is super-simple and is being used more and more often as websites feel the bite of unexpected CDN service interruptions...