LONDON — Grace Kelly was a film star before she became a princess. Princess Diana was royalty before she became a star. Meghan Markle is neither a princess nor a star, even though she married a prince and is very famous. In the age of social media, there can be no real princesses or stars, because there is no privacy and no mystique. There is no content, only surfaces, and no substance, only promises. The winners in such a media environment are unprincipled counterfeiters: actors, for instance, or politicians. Meghan Markle was never much of an actress. She is not a politician in the traditional sense, either. She has never run for office or gone through the antique procedurals of voter drives and public debates. Nevertheless, Markle, who promoted herself as a “philanthropist and activist”...