I reject the implicit assumption that Zuckerberg has an ethical duty to reduce indirect harm from hosting content, and I also reject the implicit assumption that ethical calculations ("balancing" harm) are a meaningful way to reason about ethics. I think both of these assumptions are ethically problematic. The only reason Zuckerberg apologized to parents is because other people would have punished him had he not, which does not imply sound ethics or that he agrees with what he said.

I also think normalizing an infrastructure whose sole purpose is to suppress speech is ethically problematic. Rights are rarely expanded once taken.

You're right that people post abhorrent stuff on such sites, but I prefer approaches like filters over suppression. I also think that if reducing indirect harm is important to certain people (like reducing suicide), then there are known effective ways that don't require coercive power, like the public campaigns against smoking and MADD which have both had significant impacts.

Freedom of expression comes to mind. If someone had a friend commit suicide, should they not be able to discuss their experience in public?