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.

Foundation's enough of a standard that you don't have to worry about that. It'll be around indefinitely and won't get abandoned like some hobbyist project or superseded by Bootstrap.

I think jQuery and Bootstrap are "developer ready". IE , easy to use. The easiest lib/project to use will always be the most successfull one. Doesnt mean there is no place for competition, and it doesnt mean it will always be the framework everybody use. Furthermore this is "Twitter" bootstrap. Most people have no idea what Zurb is.