It is always an option.

It starts with replying "I don't have Facebook and won't sign up to this thing" to everyone that explain that they are only on Facebook; even before you close your account. Just signalling that Facebook-only is not ok.

You're not so much being left out of everything as much as they are leaving people (you're far from alone) out of their events.

And that's part of the clever thing Facebook does to make you believe you have no option but theirs.

It took me 3 years from starting to reduce to finally deleting my account. I only wish I did it way sooner.