David Marcus, the head of Facebook's cryptocurrency Libra twitted,
Many want to pit Libra vs. Bitcoin. In my mind these two are not in the same category. BTC is a decorrelated (investment) asset. Libra is designed to be a stable medium-of-exchange. I have been, and remain a fan of BTC, but for very different purposes.
The former president of Paypal stressed that Libra is a better digital cash and payment system, while bitcoin is a long-term investment vehicle.
https://www.ccn.com/crypto/facebook-cryptocurrency-chief-libra-no-threat-to-bitcoin/2019/06/20/Their operation against Bitcoin is not what needs to be defended, because it would have even been impossible for Libra to threaten Bitcoin based on some reasons he stated, they cannot ever have same function wherein Facebook function is far more than what Facebook Libra can function for.
If David Marcus is to defend their coin against anything, it is government institutions, because I really see government coming hard on them in future for this stable coin they are about to create. This stable coin may make people to dump banks completely and start converting their funds into Facebook stable coin which will be more reliable than tether for transactions.