Amazon’s bravado was startling for its message, but also because it represents a departure from expected online corporate behavior. On social media, brands have been evolving from public-relations automatons to your cool friends. When Walmart posts about a “moment of zen … brought to you by our spring stock up essentials,” it’s just doing vanilla-flavored marketing. But when Slim Jim, the beef-stick company, sasses Steak-umm, the frozen-beef-sheet seller, over supposedly subliminal 69s in a post, it is striving to embody a personality that might resonate with customers.