I'm no expert on this subject since I haven't had a Facebook account since a few years ago, but I would imagine there are certainly some people/scenarios where something that Facebook would normally be used for couldn't be done elsewhere. Like uploading some photos. Or looking at old photos. Or messaging someone who you don't know well enough to have ever bothered getting a phone number / email from because Facebook was good enough for talking to them.

Surely not being able to look at an old photo wouldn't result in a failure of a time-dependent project.

And if an important project would depend on somebody, you would probably bother to get a phone number or an email.

As I see it, disabling facebook for a day would be much less inconveniencing than say, disabling google's search engine (of course you could use alternative search engines, but none of them come close to google's result), or wikipedia (to which there's not much alternatives, really).