It's pretty straightforward, because Instagram displays users' account number on their IG profile.

If someone has an account number of 50,000, it seems to mean that they were the 50,000th person to join.*

I have an Instagram account with a lot of followers, so I've hade a steady stream of notifications coming in about new users joining.

By viewing the profiles of people who just joined the platform, I can see where they were in line and get a general idea of how many users have signed up.

I started checking this intermittently and recording the estimates.

*I don't think this is explicitly stated, but it seems to be true based off of Zuckerberg's posts about user milestones.