Since Facebook announced their coins, I don't see that this will be a threat to Bitcoin. Facebook has a different purpose than Bitcoin and maybe it's just the adoption of their same technology.
I can't comment too far, because we haven't seen for ourselves whether later Facebook coins aren't like bitcoin? and what are the benefits for us to buy the coin. When compared, it remains a bitcoin that is targeted by investors.
The advantages of buying Facebook coins? You can only buy Facebook games and promote your ads through Facebook ads. They will make Libra like Paypal, only for transactions on their platform.