As far as I can see sending money with Libra would be quite similar to sending money with say Ethereum. Transfering ETH from one wallet to another is semi anonymous and cheap, like one US cent. However converting fiat to ETH or back at the other end costs in the tens of dollars and requires things like being videoed holding your passport for KYC and AML. It tends to take a few days to be approved. I think the KYC process costs companies like $40. So it's not a cheap or quick process if you include that stuff and fairly traceable compared to cash.

Facebook may have an advantage on KYC as it already knows it's clients through their facebook activity and so may be able to get the costs lower which could be a competitive advantage there.