In any case, on a fledgling social media (i.e. focus on sharing/remixing audio/video/images) that I built for a client, the current user registration is just shy of 2,000. The number of users who have posted anything is ~700. The number of users who have posted in the last 30 days is ~350. The number of users who have posted 3 or more items in the last 30 days is ~100. The number of users who have commented on a post in the last 30 days is ~150.
Session duration will greatly influence concurrent usage. If people use your service for less than five minutes per day then 100 daily users would mostly have the system to themselves. This means that you could have thousands of registered users and grossly inefficient code without load issues.
Stop fretting about your hardware and get thousands of registered users.
My guess is that I should plan for a concurrent user load of about 5%-10% of the registered users (both free and paid). Included in that range would be the unregistered anonymous users. I expect at the end of the day the actual concurrent usage will be much lower. Probably less than 1%. But I don't think the hardware costs to support 5-10 are five to ten times the cost of supporting 1% (probably a factor of 2 or 3).