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!
Libra coin has still lot of development to be completed before its issue and being stable currency it might be used and adopted by large number of people but still btc cannot be compared with it as it has other level like pricing factor which fluctuates and gives huge returns to investors and moreover it is decentralized coin without any control which is not the case with Libra as it is owned by Facebook and will not be able to survive in long run.