This is maybe-probably the only website I've posted on in the last 7 or so years where I don't feel like it'll inevitably fizzle out if it doesn't Take Off, and while I know there is more than one factor contributing to this, the most visible and interesting to me is that it already has a strong site culture.
That sounds obvious but I've tried a lot of new social media sites; even with the best intentions and a specific audience in mind the vibe tended to be very much like my eighth-grade class dance. There's a party set up for sure, and everyone knows what they're supposed to be doing, but they're not sure how to do it or even how to tell if they're doing it wrong.
This is a self-reinforcing layer of ice that has to be broken on a mass scale before there are enough people comfortable interacting with each other to organically develop things like site-specific in-jokes and camaraderie. Without stuff like that, every user who posts something self-indulgent, offbeat, erotic, vulnerable, etc. is making an individual decision to get off the wall, stand in the middle of the classroom, and bust a move. Best case scenario, other people were waiting for someone to start so they'd feel comfortable jumping in. But it's just as likely--if not more so--that the poster is going to stop dancing between "Return of the Mack" and "How Do You Talk to an Angel" because everyone else is just awkwardly watching them.
Cohost's design really makes this work--it's immediately clear that you're supposed to understand interactions as happening between people and other people rather than a Platonic Ideal of the User and a buffet of Content™, even if you're just sharing posts. But on a very straightforward level, as far as I can tell the site culture is as robust as it is now because the creators and early folks knew what they wanted it to be and just started setting that example.
That should probably not be revolutionary, but it is an enormous shift from the approach most sites take right now, which is to address the User as a powerful tastemaker. The existence of real people with a strong idea of the type of community they want to encourage treads on the assertion that you--yes, YOU!--are the protagonist of the site, and that the platform's only stake is in you and their role is to get out of the way of your rising star.
IMO this setup only works if you want interactions to be User -> Content, because of fucking course every person who runs a dam web sight knows what they want people to do on it. If you're pretending you don't, you have to try to steer them toward it indirectly, a thing that is much easier to manage if you encourage homogenous behavior. This has the knock-on effect of limiting a lot of the natural paths people take to expressing themselves and deciding whether they like perfect strangers enough to get to know them better.
I should climb off but there's a specific type of MMO design approach that shines a flashlight up your ass about how your character is really, truly, hand to god the sole protagonist of the story, while also making it clear that you should never do anything in-game that you haven't been told to. An easy way to make this less exhausting is to write lore that accounts for the existence of other, equally important player characters. That's what it feels like Cohost is doing.