While user numbers have plateaued, I think the story is more nuanced. It seems like the demographics of FB have been changing rapidly - what it indicates to me

While user numbers have plateaued, I think the story is more nuanced. It seems like the demographics of FB have been changing rapidly - what it indicates to me that their ratio of acquisitions to churn is decreasing, with most of the new acquisitions coming from an older demographic. So, you're both right - young people are leaving/becoming inactive on FB, while being replaced by older people.