"Critical," to me, means "what your clients/customers are paying for," or perhaps "what you're competing on with those in your product space."

Facebook's critical features are the friend network, the information page, and the wall. Facebook would be stupid to write their own HTML rendering library. They would be stupid not to make sure they have the optimum data structures and indexes underlying their relationship graph, and therefore probably wouldn't use an off-the-shelf FOAF library.