Facebook had ban crypto and ICOs ads so it really is a surprised for them planning in creating a crypto of their own however, I think it would work more if they will also used bitcoin. Creating another crypto exclusively for them is smart but using bitcoin could be more practical.
Of course using bitcoin would be better for the users, but Facebook don't care about the users
at all. As far as they are concerned, users are a product to be, well, used. All they care about is profit. They banned crypto and ICO advertising under the guide of "protecting their users", when obviously it was actually because they want to launch their own crypto. Facebook don't want their users using real cryptocurrencies - they want users to use their private and centralized crypto so they can make a profit from it and gather your data to sell.