its a good idea and if facebook create its own crypto then crypto market will boom and like facebook many big company will also use blockchain technology.
Every company can use the blockchain technology but that does not mean that they need to create a coin that is being traded in any exchange, it can be a private blockchain project rather than a public one. I do not think so that facebook will start a new crypto, but they might integrate the blockchain technology at some capacity. What is the real use of every company starting their own coin.