Relevant HN Discussion: https://news.ycombinator.com/item?id=36673040
The same way they do on Facebook and Instagram. By using the vast amount of data they have already collected.
So really, the headline, according to you should be “only not writing anything on the public web will be enough to protect your privacy”. You argument has nothing specifically to do with the Fediverse or Threads federating.
I think the point is that we should all agree to limit Facebook’s access to our data.
Federating helps them do shitty things and that seems bad.
Yes, I agree about Facebook. But over exaggerating the threat to privacy that federation poses isn’t the way to do it, in my opinion. Instead there should be a clear, well-informed and accurate risk assessment