People die eventually anyway, why should we blame murderers? The obvious answer is that speeding up a negative result is still bad even if that result is eventually inevitable.
> Domestic US political issues will not be resolved by making Facebook disappear (why does it not seem to have similar impact everywhere?)
The US system of government is much more prone to polarization with its first past the post voting and two party system. That said, polarization is increasing across the world. One of the things the whistleblower mentioned in her 60 Minutes interview is that European political parties complained that Facebook incentivizes taking more extremist political positions.
No one thinks that killing Facebook will magically fix everything, but it will at least help slow the rate of things getting worse. Think of it like climate change. Reducing carbon emissions won't retroactively fix the damage already done, but the important first step is to stop things from getting worse.