facebook dead
When someone hasn't logged into Facebook in so long it's like they're buried under a pile of old memes and expired cookies, and they don't plan on coming back anytime soon. If they actually died, you just say 'Dead', no need for extra drama.
My uncle hasn't posted since 2013. He's probably dead and doesn't know it.
She hasn't liked a photo since the Obama era. She's officially Facebook dead.
He hasn't checked his Facebook in two years. I think he's ghosting the internet.