By "pivot into mobile" I suspect the other poster is referring to Facebook Home, an ill-fated Android skin and line of smartphones.
At the time most features were designed and implemented first for desktop and later ported to mobile. He issued an edict to all hands: design and build for mobile first. Not at some point in the future but for everything, starting immediately.
Maybe this doesn't sound major, but for the company it was a turn on a dime, and the pivot was both well informed and highly successful in practice.