How is it safe? First it doesn't exist yet. Second, for a blockchain to be safe it has to be decentralized (at least to a certain degree) and have a huge hash power plugged into it preferably from different entities that don't collude together. If Facebook is going to provide the hashpower for that blockchain, what happens if/when one day they decide to pull the plug? We have seen companies as big as Facebook rise and fall, so it may happen.

Just a reminder, Bitcoin (and its blockchain) is not dependent on any single company and that's what makes it safe and valuable. Moreover, it has been around for 10 years and has already proven its reliability.

It is really silly comparing a future centralized questionable coin with bitcoin.