Now, let's roll forward in time a little and assume you have been successful enough to attract tens of thousands of users and your single database starts creaking under the load.
My general rule of thumb is to start planning for a bigger machine or splitting the database when you are over 80 percent utilization at least for a few hours a day. It's good to have a plan earlier, but now you have to start doing something about really carrying out the plan.
There are a couple of popular ways to grow database-backed systems. Depending on your use case, not all ways will work.
If you suspect that you are near your top load for the service ...