Most of the people don't have a clue how the Internet actually works (and they don't need to), and that doesn't stop more than a billion people to use Facebook daily.
This actually illustrates the point beautilfully, what would happen if, suddenly, one billion people couldn't use Facebook anymore?
The idea that this would make the internet one fifth less useful is fucking ridiculous on its face.
If, suddenly, one billion people couldn't use Facebook anymore, then it would be because of a literal apocalypse. Facebook can't just disappear overnight. Even if they were to go bankrupt or banned or anything else, there would be an announcement, letting people leave for other services before they do disappear.
Also, Bitcoin is different in that, while Facebook severs can be shut down tomorrow (but highly unlikely), it's literally impossible to just shut down Bitcoin servers without bringing down the whole Internet in the process.