In a video released by Facebook, the company’s chief executive, Mark Zuckerberg, and his wife, Dr. Priscilla Chan, explained why they have decided to devote nearly all their wealth to charity. L.L.C. stands for limited liability company, which is a specific type of business structure. The use of such an entity, revealed on Tuesday when the couple said they intended to give away 99 percent of their shares in Facebook during their lifetimes, shines a spotlight on an atypical but increasingly popular financial vehicle in philanthropy.