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I have done the same with Instagram and there is no background network activity.

I don't use any Facebook services, but when a friend visits my place I see background network activiy in my PiHole from all Facebook-owned apps.

Did you to examine what is trigggering it? Is because of a shared IP-address or a shared physical location?

Not really. It's just that tons of Facebook/Instagram domains show up in the PiHole admin page.