Mediaboss Marketing

The simple answer is no.

Easiest analogy;

You are the host of a party. You then invite 11 other people, for a party of 12 people. One member of your party (let's call them W) is not liked by 3 other members (we will call them 'the clique'). During the party, 2 members of 'the clique' talk about W to every other member of the party to the point where some of the other guests dissociate from W. Can blame W blame you for defamation?

I would say yes if any one of these conditions could be met:

(A) You would have to have known about the nature of the relationship between W and 'the clique' prior to inviting all of them to the same party.

(B) You would have to have knowledge of and ignored the actions of 'the clique' during the party.

(C) You would have to have knowledge of and ignored any request W made to limit 'the cliques' actions.

In this situation you are social media, W is a public figure, 'the clique' are bad actors, and the other guests are random users.

Condition A requires websites confirm identities with background checks (Think Chinese internet). This is possible, but expensive for small private sites (like JapanToday) and could be seen as invasive for those who could pull it off (like Alphabet/Google or Meta/Facebook).

Condition B requires that the websites not moderate or monitor what discussions happen between it users. Users are not free to share any kind of content between other users (hi moderators, watch this get edited) on any website. Facebook will take down content it does not like, JapanToday will lock comment sections and delete posts deemed inappropriate.

Condition C requires websites to receive complaints and not act on them. Facebook has historically had no direct phone number but there is a reporting system and again they will delete content. JapanToday has a reporting system in place and does delete comments.

While there is an argument to be made about the decision-making process and timing of editing, moderating, and presenting content, it is hard to argue that a third party can act in a way that exposes a media company to defamation.

More:
Do you think media companies should be held liable for defamatory third-party comments on their Facebook pages or discussion boards? - Japan Today