To be honest, it can be a wild card. There are many scenarios that could happen. All of which are hypothetical - until they are not:

1. Facebook coin is integrated on its social network.

As a result people wishing to send money to their loved ones can do so via messenger, which is really a no-brainer. FB then corners this market, the people who don't give a shit about blockchain technology, speculation and such that is endemic to the bitcoin ecosystem. The billions of users are happy.

1.A. Facebook coin does not take the 'usual crypto' route and refuses to integrate to the crypto exchanges.

It will become a de facto fiat/or stable coin that can't be traded. Can they do that? Yes they can, if they limit the technology within a closed loop in Facebook.

I'm not so much into the technical aspects, but I surmise that they can do this. If they do, Bitcoin will become relegated to a core niche community, and fb coin will rule.

1.B Facebook coin allows open integration into the overall cryptocurrency ecosystem.

Chances are it will become tainted with regulatory uncertainty BECAUSE it will be subject to speculation. I would surmise that this has less than 50% probability, knowing full well facebook's regulatory concerns. The last thing it needs is the SEC, FATF and all those interagency task forces looking into their business when there's already talk of privacy and anti-trust issues.