I don't think there's a inviolable ratio between active users and engineers, but you do absolutely need more engineers as your user base increases: you need mor

I don't think there's a inviolable ratio between active users and engineers, but you do absolutely need more engineers as your user base increases: you need more people on call, more people maintaining the servers you're running (especially if, like Twitter, you run your own DCs), more people doing QA and reliability engineering because of increased load, etc.