Facebook and Twitter provide accountable moderation. Parler doesn't and instead claims to assign moderation to random unaccountable users instead. This is according to their CEOs own interview earlier this week.
You can look at www.reddit.com/r/parlerwatch to see examples of extremism on Parler.
Doesn’t 230 protect websites from moderation because it would be completely untenable?
Some of the most extreme speech I’ve seen have been on Facebook and Twitter. However I don’t hold them responsible since they’re providing communication infrastructure.
I'd have to read up on the exact extents of 230, but it's largely related to lawsuits and does not preclude platform TOS from holding things to a higher code of conduct.
Additionally I believe that 230 doesn't apply to knowledgeable hosting of illegal content. So if inciting violence is a known element on a site, and the owners are aware, then I believe (not a lawyer) that they can't hide behind 230.
Also I do not believe 230 prevents lawsuits against hosting services necessarily. It may make tech companies liable for providing a platform to a service that is behind 230 but they themselves can't claim that.
With regards to Facebook and Twitter, they have significantly larger user bases and active moderation. So definitely not perfect but they can remove content that violates all the ToS they may be beholden to. Parler does not provide such a thing, and doubles down on not doing so. So a lot of content gets removed from FB/Twitter, and if stuff doesn't due to slipping through the cracks, they still abide when notified of transgressions.
Additionally, as noted elsewhere, parler does moderate by removing content that the community dissaproves of, but that is legally in the clear, white leaving up what is largely considered discussion of illegal activity. Therefore they are tacit in those conspiring events.
Parler openly allowed the planning of this violent anti-democratic insurrection and has been deemed bad faith in their moderation.