For as long as I've known the internet, I've tried to restrain myself from creating a Facebook account. I was successful until recently, when I needed to make an account in order to run a Facebook ad. I avoided Facebook on principle: I did not want to contribute my information to a company whose business model is the monetization of identity. The ad itself wasn't particularly important either, but I caved and decided to sign up anyway. I wasn't going to visit Facebook for any reason other than to manage my ad campaign, so I considered my violation of personal principle justifiable. The process of signing up itself was mundane: I entered my name, my email, and my phone for verification, and I was in. I was greeted by Welcome to Facebook! and a list of people that Facebook's algorithms suggested that I add. And, for a site that I supposedly had never interacted with, its suggestions were surprisingly accurate.
There they all were: my family, my old friends from school, my best friend, and some other acquaintances. I X'ed out the ones I didn't know, and others would appear, gradually becoming more accurate. By giving the suggestion system feedback through my X'es, I was giving it valuable information about who I knew (and didn't know). As a result, the suggestions became even more accurate than they originally were. So how could a site that I had presumably never interacted with predict my social connections so well?
The answer is that Facebook isn't opt-in. You don't need to have a Facebook account for them to track you. Facebook's tracking extends far beyond its own site at facebook.com. Instead, it reaches to every site with a 'Facebook' share button, every site that you visit directly after being on a site with a 'Facebook' share button, as well as every site you visit immediately before visiting a 'Facebook' share button site.
But it doesn't end there. Every time you are tagged on Facebook, mentioned on Messenger, or even appear in a photo, Facebook knows, even if you don't have a Facebook account! This is by no means a complete explanation of every way Facebook tracks you—to make such a list would be impossible, because not all of Facebook's tactics are known.
What is interesting about most of these methods of tracking is that they do not require any action on your part. Instead, they largely operate by means of association. If you are tagged in a photo on Facebook, it creates a connection between you, the poster, and everyone else who appears on this photo. Then, if you appear in another photo with some of the same people, Facebook will then assume that you are in a shared social circle. Then, it will be able to fill in the gaps in its picture of you with the data it has on others.
So unless you have no social connections whatsoever (a situation in which Facebook's tracking operation would likely be the least of your worries), then Facebook's ubiquity among your peers makes it almost impossible to escape its grasp.
While I have only touched upon a few very specific examples of Facebook's activities, they very effectively demonstrate their involuntary nature: you do not need to divulge your personal information to Facebook yourself—other people will inevitably do it on your behalf.