They're not though. The author worked for Twitter when he released the first version of Bootstrap, but Twitter never actually used it in production. Twitter doesn't promote it anymore either; BS3 got moved to twbs/bootstrap on GitHub, and the new marketing site doesn't call it Twitter Bootstrap anymore.
ZURB doesn't have a huge consumer product, but they're a pretty big name in design, and they have half a dozen people who regularly work on Foundation/Foundation support.
Backing up what ricefield said, responsive design is making something that works well in many different viewing contexts (desktop, mobile, tablet, smart TV, etc). http://zurb.com/word/responsive-web-design
Placehold.it might have been down. They're great for development, but a lot of people were probably opening the templates and sucking up their bandwidth yesterday.
ZURB doesn't have a huge consumer product, but they're a pretty big name in design, and they have half a dozen people who regularly work on Foundation/Foundation support.