It’s likely there’s a root cause, like a fiber cut or some other major infrastructure issue. But, Down Detector doesn’t really put a scale on their graphics, so it could be that it’s a huge issue at Meta and a minor issue that’s just noticeable for everyone else. In that case, Meta could be the root cause.
If everyone is mailing themselves their passwords, shutting their phones on and off, restarting their browsers, etc. because Meta wasn’t working, it could have knock-on effects for everyone else. Could also be that because Meta is part of the major ad duopoly, the issue affected their ad system, which affected everyone interacting with a Meta ad, which is basically everyone.