A thing that stands out to me is that it worked at every size.
On day one, it was just the Odeo team that Twitter ended up spinning out of, maybe 15 people. And it worked that way as a group messaging thread where we felt more connected to the people we worked with.
Then we let in close friends and family with a stern warning from our CEO not to let anyone from Google see it. So that was 50 people and maybe each of us had 3-4 close friends on the platform.
To someone else's point that the community already exists, you're just building the communication platform: this was still when Facebook was locked down. I invited in close family (existing community) and learned stuff about them that sounds trivial but was meaningful to me (communication platform).
Then the community ballooned again to maybe 500 people and suddenly there were interesting tech luminaries to follow.
And on and on.