a) Facebook has less users not more, while they always emphasize their user growth I come across more and more people in real life who simply don't have a Facebook-Account, and those are social people. I am careful about basing too much on my own experience ofc, but this was very seldomly the case 4-5 years ago and now happens to me very often.
b) More and more users are disillusioned with Facebook to some extent. Due to some minor reasons I keep a Facebook account (various purposes, among those domain-specific groups to get questions answered and log in maybe once a month) and I see among my peers many who aren't really using it for much anymore. The occasional photo update with minor interaction via comments, the occasional posting of a music video, or the few users in between with heavy political activism on their timeline who apparently don't realize that next to no one will read it anyways and sigh about that crap. Facebook simply isn't cool anymore, so many don't care for most of its features and instead all they want is something to easily chat with people, aka WhatsApp.
c) Facebook certainly has trouble with attracting younger users and this won't stop. A social media platform where it takes a few days after account creation til your grandfather sends you a friend request and you have to answer to your parents about what you just posted? Definitely the cool place to be. Similar things probably can be said about WhatsApp to some extent. WhatsApp is what used to be normal SMS functionality in a way, while probably everyone has it cause everyone has it, it certainly isn't something exiting anymore.
d) Those who in my opinion over-estimate Facebook's role grossly under-estimate how fragile companies are and how easy (in my opinion) something like Facebook could start to fail. It happened to companies before, it will happen again. I don't know when, I don't know exactly why (though many possible reasons are viable). Everyone being on Facebook instead of just your 10 coolest friends might be what kills it in the end for young people for example, who knows what comes along and might attract user growth. The mobile market has made that certainly more dynamic via Apps, it's easier for a new chat App to gain traction than for a new social media website and WhatsApp with 1+ Billion users might not be bold enough to explore what the next big feature is people yet don't know they want. I know Facebook owns WhatsApp, I know Facebook invests in VR, I know their revenue is extreme and they have money stockpiled, I also know that isn't a guarantee of anything...
After thought: Anyone believing in the mighty power of Facebook should put their money where their mouth is and invest heavily in the stock, if Facebook is this monster of a multinational all-controlling company led by this young genius, a company with only growth ahead and such a minimal risk of failing then it shouldn't be a tough decision to make right?