DT is right, most sites have plenty of inactive users. People make accounts then forget about the site, or maybe they put in a fake email and can't confirm it to be able to post.
Also, not everyone wants to post all the time. Maybe they just want to learn from the questions that have been asked, or maybe there aren't any questions they can answer.
Also, it's way more than 40 people. Here are some statistics:
- 130 users have posted something in the last week.
- Over 220 users have logged in in the last week. This is not necessarily everyone who has visited because some people stay permanently logged in.
- Nearly 450 users have either posted or logged in this month of May.