Facebook Inc.’s plan to create a set of digital currencies, Libra, is a big idea. If a third of the social network’s 2.5 billion monthly active members use Libra, it might be in more (virtual) pockets than the U.S. dollar. On the other hand, it could flop if it turns out to offer little more utility than existing payment apps, or it might not ever see the light of day, if it’s strangled in its crypto-crib by regulators appalled by the idea of a potentially dominant global currency that could ope