"I've not quite figured out yet why we need *any* cryptocurrencies."
Because we think we're being screwed by the existing financial providers and regulators and would like to get out from under the heel of governments that use regulation as a method of monitoring and control.
However, it's difficult to think of anybody less suitable than Facebook to run an alternative. The Mafia, maybe ? No scrub that/ The mafia's violent reputation makes them scary but they seem archaic, limited and trustworthy compared with Zuckerberg. More scam rackets than global control.