It wouldn't plateau because you get throttled across the board over all your posts and significantly so. Your posts, old and new, simply don't get promoted into

It wouldn't plateau because you get throttled across the board over all your posts and significantly so. Your posts, old and new, simply don't get promoted into timelines anymore. As a result, your traffic from FB will rapidly drop until you pay them off, that's just business if you want to spread on FB.