Thinking about it, Facebook has a few things going for it. It's become an important communication channel outside of immediate family and Best friends. Collecting email addresses across multiple web mail accounts, that's a chore. Facebook has made messaging people easy. That convenience has got staying power, especially as in regards to mobile communication.
Teens aren't interested in Facebook anymore? Doesn't matter, they have no money to spend.
The sweet spot is advertising to older people with jobs.
The balance they need to find is the amount of directed advertising they can force on a user, without making them abandon FB... or just use it strictly as a messaging tool.
Scare off users from reading/liking/posting items in their feed, lose the other revenue stream: mining and selling social trend data.