I think that they would affect the crypto market in a good way they would attract more investor since they are already well known and making more publicity for crypto.
But I don't really think that most of us would be interested in those centralized alt-coin and we would just use it for trading to gain profit ,
Most of us wouldn't really use it for it's sole purpose just like most of the centralized crypto .
I'd say, they will aid in crypto mass adoption in one way or another. Because of the huge number of their users, the adoption might come easy for them. Many people are still ignorant in dealing with crypto, and most of them thought that it's too complicated to do a simple btc transaction. With the deployment of fb coin, their dev team might develop a process that is dumb-friendly and so their quick adoption begins. We will know such development in the near future. Let us see...