The more likely reason, however, is that assuming FB or social media in general increase polarization, it would almost certainly worsen it in the US more than anywhere else because of the US’s fairly unique 2 party structure combined with the primary system, both of which would almost certainly exacerbate any polarization effects caused by an external factor.
But the fact is that this isn’t the defense Zuckerberg thinks it is. In fact, it may even suggest the absolute opposite.
Facebook has never been as popular outside the US as it has within it. The best indication of this fact is FB’s $19Bn purchase of WhatsApp which was largely driven by the fact that FB Messenger was basically only popular within the US, with the rest of the world preferring WhatsApp, which was also an indication of how FB’s network was simply not as entrenched outside the US as it was within it.