But that wasn't my point. It's not that I think that Facebook or Google cannot scrape Fediverse platforms/instances, it's that even if they do, they cannot serve targeted ads based on our activity here.

We have different definitions for privacy. Since I'm active here, it should be clear that to me private doesn't mean hidden. I like how the EFF put it, in their article on the Fediverse:

[T]he default with incumbent platforms is usually an all-or-nothing bargain where you accept a platform’s terms or delete your account. The privacy dashboards buried deep in the platform’s settings are a way to tinker in the margins, but even if you untick every box, the big commercial services still harvest vast amounts of your data. To rely on these major platforms is to lose critical autonomy over your privacy, your security, and your free expression.