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.
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 It'll be around indefinitely and won't get abandoned like some hobbyist project or superseded by Bootstrap.