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.