The analogy to Cixin Liu's "dark forest" concept also seems apt and enlightening. (Basically: it's a game theoretic response to the Fermi Paradox that suggests that any sensible galactic civilization would avoid making contact with any other civilization and attempt to stay hidden.)
More and more over the past few years, I've seen my own social circles migrate away from public forums like Facebook and Twitter and into private WhatsApp group chats, Discord groups, and so on.
In some cases these are groups of people I know in real life, and in some cases everyone is anonymous or pseudonymous.
And there is definitely an unspoken rule of "don't unilaterally invite anyone, don't advertise that this group exists, stay hidden, we like what we have going on here."
It's in some ways a reversion to the style of older private Usenet/BBS/IRC channels, but in other respects it's a lateral move. For one, it's still mostly happening on centralized platforms.
What I think is interesting is how our media ecology (in the sense of media as means we communicate and express ourselves, not "mass media") is an interplay between these big public spaces and a proliferation of smaller private spaces. It's not _just_ a dark forest, there's also a bright canopy into which people emerge, forage, and carry back down into the forest.