If about 50% of facebook users decides to try facebook coins because of the companies reputation, won't the demand of the price make the price surge and hence call investors to convert their bitcoins for facebook libra? It is a great treat to Bitcoins and Ether.
If 10% of facebook users use their coins it would be relatively big. People already use venmo and zelle. It doesn't have to be a good cryptocurrency; it just has to be easy to use.
It won't compete with btc, it will just stifle its growth.