A lot of good points being made.

A community isn't just its leaders, but a community tends to follow its leaders. When the admins are friendly, the community tends to be friendlier. When they are toxic, the community tends to be toxic too. I have seen plenty of sites die because the admins were too busy or otherwise inactive. I've seen plenty of sites die because the admins and their friends wouldn't really plot with others. And let's face it: if a community has gotten too toxic and the admins aren't happy with their site, it might just be time to let it go. RP should be for the enjoyment of everyone.

We shouldn't dismiss the impact of admins though. Successful sites have active admins to help foster an active community, and systems in place to encourage roleplayer interaction. I was recently on a site that was so welcoming to me despite being a very sparse roleplayer. People would regularly go out of their way to chat with me and tag me to plot. That was just the site culture, and it started from the admins' efforts. It helped that people were rewarded onsite currency for threading with others for sure, but they would have received the same rewards if they stayed inside their cliques. There was a genuine desire, both from the members and the admins, to include new members in both personal and site plots. I'm sure it would've been very different if the admins weren't a driving force through chatting all the time, hosting various events onsite, and recommending characters to interact.

You can't just throw a bunch of different people into a room and expect all of them to get along on their own. Sure, some of them will, and you'll have your various cliques. But if you're the one who invited them there and want them there, you might as well help integrate the groups, and soon other people will follow suit.