Facebook's rumored crypto coin is unlikely to replace bitcoin. Other blockchains have developed in response to the shortcomings of bitcoin. Even bitcoin has been forked to deal with the shortcomings of bitcoin.
The shortcomings of bitcoin are that it is expensive to mine, is very volatile in price, does not handle smart contracts, and has a difficult to remember address. Despite all the lessons learned from bitcoin on what not to do, it continues to dominate as one of the top cryptocurrencies. Thus, another alt coin would not make much difference.
Some considerations for Facebook would be whether to host and own all the nodes, whether it would be a stable coin, be able to do contracts, and how accounts would be addressed to make it easier for people to use than bitcoin. Their challenge is to make their currency accessible to non-technical people and to be able to control it.
Facebook could use any currency they want if they intended their blockchain to be decentralized. However, if they want to control it, then they would need to make their own. Thus, having centralized control of the blockchain goes against the idea of bitcoin, which means people would not abandon bitcoin to jump on a centralized blockchain.
If, on the other hand, Facebook made their currency open-sourced and decentralized, then perhaps you may see greater use from the crypto community, which is different from the average Facebook user. However, it is unlikely to lure people away from bitcoin entirely. What makes bitcoin useful is its scarcity and resistance to scaling, both of which are of little use to Facebook. They need a coin that can expand and scale to billions of users. This is a different need than what bitcoin fills.