Here's a detailed article on this topic by CNBC:
cnbc .com/2019/06/18/the-difference-between-bitcoin-and-facebooks-libra.html
In my view, the biggest difference between Facebook's Libra and the Bitcoin is that while Libra is a Stablecoin, Bitcoin is not!
Bitcoin is a fully decentralized currency while Facebook Libra is centralized shitcoin!
Bitcoin is open-source, backed by the people while Facebook Libra is not open-source, backed by Facebook, PayPal, VISA and so on.