The "Zuckerberg defence".

It's OK to pirate a massive amount of books if you're not reading or sharing, but rather just training an AI.

I don't know where I stand on the issue but it's interesting Facebook has been known to block PB links while Google seemed to refuse requests to do the same

What are peanut butter links?

I'm guessing Pirate Bay

Oh, I don't recall seeing anyone sharing Pirate Bay links; why not share just the magnet uri?

Or is it about sharing the domains of mirrors?

Yes

And by the way I prefer Google's approach in this particular case

Zuckerberg strikes me as far too adaptive, too fair weather