This is basically the point of decentralized systems. Foor bitcoin to work it relies by protocol on whoever is online, so effectively everyone using it would have to go offline for it to fail. Additionally, bitcoin really doesn't get updates that would dramatically change how things function enough for their to be an error introduced that would take things down. In comparison, FB and friends rely on a handful of servers that typically can be taken out by one poorly planned software upgrade.

This is a gross simplification, but the basic idea is that more servers and diversity of the servers (which you get from decentralised protocols) should lead to crashes not taking out the whole network.

Facebook's more akin to an exchange for Libra, isn't it? There's 28 companies partnering up for the system.

They have one wallet. But there is no reason for a wallet to depend on facebook being online, and presumably there will be 100 points of failure for the ledger on launch

Nope, that's the beauty of decentralized network, it never goes offline while there at least some clients connected. You could disrupt the network with other ways though.