Keep the needle pointing north. Towards the center of that dial.
Too decentralized, and you can't find anything. Nobody uses it.
Too centralized, and censorship takes over. Nobody can speak freely.
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You can think of the golden age of blogs and search as an example of both. Search engines formed a centralized hub with blogs, forums, etc. forming the spokes. For a while that worked well before it was degraded by spam and consolidation of disparate forums etc. into a handful of major platforms (fueled partly be acquisitions).
In economics, a market needs several reasonably strong businesses to get price competition. An EU study indicated that the minimum number is about 4. Below 4, price competition seems to disappear and you have oligopoly, or, at 1, monopoly.
In areas where there's no inherent effect like distance to stop centralization, markets tend towards oligopoly. Look at the number of browsers, the number of big banks, the number of cellular phone companies, and so forth. They're all between 2 and 4. The stable state seems to be around 3 big players.
This probably applies to social networks. There's only so much attention available.
(It is, of course, fundamentally impossible to keep people from indexing a default-open network, but if one does it, one does not advertise doing it outside the service-supported mechanisms).