I believe the Facebook business model can be safely stripped down to the humane kind while still keeping viable. The revenue it can generate by displaying organic ads without excessive user surveillance (analyzing a user's likes and the kinds of ads they tend to click is ok, spying on every website they visit is not), without messing with politics and without selling data about the users to 3rd parties can still be enough to cover all the relevant expenses and generate profits (probably a way more humble than now yet positive) for the investors.

Facebook can also employ the experience of it's Russian clone vk.com and let users share music (sharing the music that you like and that expresses the mood you'd like to share is a great way to communicate and many people love it!) while requiring those users who'd like to play long playlists in background pay a subscription fee.

A number of other premium features that can add value to a subscription comes on to my mind as well: invisible mode (I'd pay for it immediately - I really hate when others can see if I'm online or if I've read their message, I have been actively hating this for years, this is infuriating yet I don't mind if solving this problem would cost a dollar or two per month), posts promotion, additional typesetting features, profile promotion (top-suggest everybody to add me as a friend), verified status for whoever wants it (not just celebrities), voluntary additional surveillance/curation (for parents willing to protect their underage kids) etc. Introducing a cryptocurrency dedicated to replace old-fashioned likes (e.g. every time somebody likes your post/comment you get a coin) yet traded at a cryptocurrency exchange and accepted as a way of paying for the premium features can be another opportunity. Organic possibilities are limitless.