This is indeed a very interesting question. Myself I have been wondering as of the day I started how Steemit is doing? Based on research by @paulag we know the churn rate is pretty bad. @arcange his data shows that as of 2018 the unique active users per month are decreasing. This data shows were talking of a maximum of 127k unique active users. So we can be 100% certain that it is a milestone to reach 1,000,000 accounts but that this is not synonym for active.

And like you already said yourself, there are at least a few bots and users with more than 1 account. Those should be deducted from the 127k unique active ones. Perhaps this is the only thing we need to know? And if so it would be quite easy to determine how many unique active HUMAN users there really are. Only question that then remains is if this data can be queried? Bots probably yes, but double accounts probably no. Cause how to determine?

I will do some more thinking and research about this in the coming weeks.