I don’t know that your comparison to Facebook holds water. Firstly, Meta’s employees are spread over three divisions: Apps, Platforms/Infrastructure, and Product Services (ads, strategy etc), where Facebook itself is just one part of the Apps division. Even assuming that Facebook occupies 50% of Meta’s total workforce (likely a massive overestimate), that brings us to around 30k employees for 3billion users, or 100k users per employee. That gives you about 0.5 FTE for your instance.

More importantly though, the job of administering a mastodon instance isn’t really comparable to the job of engineering a social network, so taking a Facebook’s salary or user numbers doesn’t really give us much actionable data. We don’t know how many Meta employees are directly involved in administration of Facebook, or how much they’re compensated.

Ultimately, it’s about what your users are willing to pay. If you can persuade all 10k of your MAUs that $9/month is worth the value they get from your instance, then go ahead. However, I suspect that you’ll be lucky to get even 1/10 of that.