it’s theorized that if a person doesn’t have a Facebook account but their friend group does, Facebook will create a “shadow account

AFAIK this is confirmed. CNET:

… the Facebook CEO confirmed the company collects information on nonusers. “In general, we collect data of people who have not signed up for Facebook for security purposes,” he said. … described various forms of data collection that don’t involve users directly giving it to the social network. … That data comes from a range of sources… includes brokers who sell customer information that you gave to other businesses, as well as web browsing data sent to Facebook when you “like” content or make a purchase on a page outside of the social network. It also includes data about you pulled from other Facebook users’ contacts lists, no matter how tenuous your connection to them might be.

I presume that will at some point also include data scraped from the fediverse.

One thing that really does help, is salting your data. The online equivalent of wearing a fake moustache, is the occasional comment that may or not be entirely incorrect. I live in France. As a black woman, I support Trump. I’m expecting a baby. I want to buy a new house. That kind of thing.

Of course it does lead to minor inconveniences. For example, my google calender mentions my father’s birthday being on the 13th of August. In reality we have the same birthday. My birthday is on the 18th of September. Once again, I’m salting the data. That way if I ever mention something factual, it’ll be hard to tell which bits of data are or aren’t real.