Now think about the average chat app like Slack or Teams. Those have a very different read-write ratio. Some things can be cached for long periods of time, but many other entities become invalidated quickly, and are subject to intricate permissions policies.
As for sharding, Slack uses Vitess: https://slack.engineering/scaling-datastores-at-slack-with-v...
> most people on Twitter aren't writing Tweets
I don't know how true this is. Most people in my corner of twitter tweet several times a day, some of them have rapid-fire conversations in the replies. And that cache-busts (or at least appends to) a whole lot of people's feeds, just like it would people's chat history (while in both cases, almost all writes are appends)
But idk, I'll stop speculating here because I'm not basing it on much