So the unprofitable users are leaving
The kids, being kids, are all fluttering off to the next fashionable location. Does that really matter? they don't have much in the way of disposable incomes, anyway.
> An older Facebook demographic ... an audience with deep pockets,... consuming content and completing transactions
and that's where the money lies. I once read an article about tourism from the tourist manager of a large provincial town. The view there was that it's far better to have 1 visitor who spends £1,000 than to have 1,000 visitors who spend £1 each. Maybe if FB takes, or is forced down, that route it wouldn't be such a bad thing for them.
However, if I was Mr. FB I'd hardly be worrying. So long as the share price holds up until I can cash-out that's all that matters. After that no doubt he'll stand down so he can spend more time with his money and maybe (follow Gates) into doing some good, or colonising outer space, or buying a presidency or two - either for himself, or his friends.