I have my FB set to "friends only", so I can make posts which can't be linked from Google results or elsewhere. Generally, anything which is either less-general-interest or more-sensitive than twitter, but which I want to post in general, vs. to a specific crowd. (Even more public/reputational than twitter would be HN submissions; HN comments generally are on-topic, and more on par with Twitter for sensitivity, although down in a conversation not as generally relevant.)
I also have a few "secret fb groups" where we discuss various things.
None of this really requires extreme privacy; it's just "please don't quote this in public yet" or "this might be too boring for the public."
Since I've curated my friends well, I tend to see pretty interesting debates about: 1) guns (which is fun because especially the UK friends are anti-guns, and others are in the industry, and it remains civil)
2) crypto, security, and hardware stuff (including my own startup)
3) political issues (generally "good governance" which tends toward non-partisan, but also generally libertarian stuff)
4) major world events (e.g. I have friends from Ukraine, and when Afghanistan stories come up, I have friends I worked with in Afghanistan, and some who are in Afghanistan now)
FB Messenger isn't secure/is archived by FB for unknown future use, but is remarkably convenient. If nothing else, it is better for "presence" than any of the encrypted chat stuff I use for content, so either non-sensitive discussions there, or "can we talk on OTR/irc/etc.?"
It's also still my favorite photo sharing site for photos restricted to friends. My workflow involves Lightroom and then publishing to Flickr (public), FB (friends), 500px (pro). (although my real workflow is "archive to a drive and sit on them until I have time to edit...I have about a year backlog right now).
I don't use Dropbox or WhatsApp.
You've just described the typical use-case for what most people use FB for. I don't think FB was ever about sharing with a million people, just whoever you friended.
LiveJournal (RIP) is what trained me in all this behavior.
WhatsApp has no penetration in any social group I've ever been part of. I'll be disappointed if it does displace Facebook, because I so much prefer being able to transition a conversation to my laptop.
Just add the people you actually talk to on Facebook. Your friends and family, guys from your gym. Create a group with your Chess Club in it so you can discuss chess openings.
I don't think any one apart from my friends can see what I post on Facebook, not sure you can even share your page with millions of people unless millions of people follow you.