i think it is not a true news someone spread that fake news so there is no any possibilities to invent the facebook coin.
Indeed, Facebook doesn't need Blockchain into their platform because people are still using it everyday but Blockchain technology are being used as the better alternative as another payment for their products and services because this will give more accessibility and flexibility for every business now.
Some social media and other internet platforms are now adapting blockchain technology innovation in order for them to offer wide ranges of services.
Facebook can adapt blockchain but I think they're not due to people are still using their platform whether they adapt blockchain or not.