I think you just give people premature fear. Do not be afraid of what is unlikely ever to happen. I am sure that the Facebook coin on the contrary will have a positive effect on Bitcoin.
It will bring more promotions in regards to crypto industry, bitcoin will get more attention when this project start to work inside, curiosity will bring more people inside the market, the industry might be lift as more investors to come around and help, we ain't sure how or where the market will be heading if this project will be a big lift or just another ordinary crypto that will be formed.
Or Facebook will use their massive wealth and lobbying power to create laws pushing other cryptos out of the legal-sphere and into the illegal. Many large companies use this exact strategy to achieve market dominance.