One thing we might be discounting though is other nations. (I assume you live in N.A.) I don't know how widespread Facebook is elsewhere. It could be that Facebook gives up on N.A. market and let Insta handles their interest there and focus itself on emerging markets.
I live in Europe, not N.A. We use messengers a lot here and many people still use Facebook. I've heard that people use SMS and MMS much more actively in the USA (I have never used MMS and have stopped using SMS years ago).
I don't use Whatsapp nor was I ever invited by someone to use it, but I heard it's basically the iMessage equivalent in the EU and in other places where the iOS market share is lower.
Don't know any of my friends who use it actively, and the only time or reason why I log into Facebook is when I have to talk to people who are only on Facebook.
If I were Facebook I would be seriously afraid at this point. The whole thing could be the next Aol or Yahoo. Things go down the drain pretty quickly in products like this.
The biggest change can be noticed, if young people don't create new accounts there. And I think we have already arrived at that part.
UPDATE: The only thing I used to use Facebook for a couple of years ago when I had more spare time was finding events to attend for fun. Does anybody know a good alternative? There probably is a special app just for this. Another useful feature of Facebook seems to be the groups directory: there are groups in WhatsApp and Telegram but they are harder to find - isn't there a project building a directory of them already?