There are no feasible technical justifications for doing so. The big touted benefit (caching) is a non-issue IMHO, since it only helps the first page (if you h

There are no feasible technical justifications for doing so. The big touted benefit (caching) is a non-issue IMHO, since it only helps the first page (if you haven't visited the site before, at least) - after that, even if the files had been originally delivered from your website, they'd be delivered from cache either way. As a developer, you might at some point have to develop on a site while not having an internet connection - good luck with that. Even your local copy is going to throw up. Saying that your site is more likely to be down than Facebook's site is also a false positive. If your site is down, nothing works, obviously. If Google's CDN site is down and yours isn't - your site still won't work. There is absolutely no reason to add one more point of failure to your site. Many of the 3rd-party sites used to load JS and even CSS do not have the technical staff or knowledge of Google or Facebook. The main reason I actually see this being done is due to laziness. No need to go download the file locally and maintain it. No need to make sure the server is setup properly and sending the right cache headers. Just copy/paste a link and forget about it. Who cares?