My theory is based on the 1:9:90 internet “rule”, meaning that you have a core group of people, typically 1% of the users that are highly active, some further 9% that post & comment occasionally and the vast majority of 90% only lurks (but might share a link in their social circles).

Taking that into account, I think why some sites fail and others not is mainly about the 9% occasional posters. If the 1% hard-core users (due to ideology or lack of moderation) manage to alienate these “casual” users then the site goes into a death-spiral that is hard to recover from.

So besides stricter moderation, I think the hard-core users of more left leaning communities are also typically less offensive/toxic then those of some other communities who often thrive on 4chan like meming and in-jokes that really only work for the hard-core user-base.

I think it’s also depending on whether or not the network is able to reach critical mass: a sufficient number of adopters of a new idea, technology or innovation in a social system so that the rate of adoption becomes self-sustaining and creates further growth. Now to reach critical mass for a social media network the following factors are important: money, hype/publicity/marketing, quality of the network and timing. Social networks often fail due to lack of money and being unable to generate enough publicity. Kind of a Catch-22. Federated networks may be able to avoid the money issue by spreading the burden of the costs over many instances and not having to satisfy shareholders. This can contribute to their chance of survival in the long run provided that a opportunity comes by where one or multiple incidents generate enough influx of new users.