Agreed, in my original post I mentioned posts per day as a metric thats slightly closer to actual load. But its pretty hard to throttle number of posts, so I think users is going to be a more useful value.
If a server has really active users they can set a lower target number of users.
Eventually it would be nice to have a standardized way of migrating and backing up accounts to make it possible for one server to offload its existing users to another server.