Extremism itself is a vacuous term and normative. Anyone calling for the end of segregation, the end of aparthiad or for gay marriage was once "extremist". So yes it is perfectly A-OK for actual extremists operating under a bare minimium of discretion.
That's a very disingenuous take, but those seem to be the norm for you. People calling for an end to apartheid have not been historically labelled as extremist (other than by apartheid regimes themselves) because apartheid has not been a norm in recent decades, and because the causes you mention all involve the expansion rather than the abridgement of rights, upholding a long-established egalitarian norm.
But to make you happy, I'll qualify my statement, and say that Facebook has been A-Ok with violent and frequently genocidal extremists organizing on its platform.
