According to Wikipedia, there were 15 million registered users on RG (as of April 2018):

https://en.wikipedia.org/wiki/ResearchGate

Yes, it would be helpful to know the participation rate of academics including a subcategory reporting. If the RG-team is reluctant, I guess we can estimate totals by e.g. catch-recatch.

If the data were accessible, this could lead to a useful and short empirical research paper.

Well public data per member is available, and presumably scrapable, on the front offering the factors you asked for. On institutional and departmental level, RG provides a fair amount of statistics including network statistics. But I think the aggregate should be published, which I believe not to be the case.

Many thanks for your helpful response.

The problem that remains is how many academics are registered at RG at any given time.

According to Wikipedia, there were 15 million registered users on RG (as of April 2018):

https://en.wikipedia.org/wiki/ResearchGate

I would like to thank all the academics who have provided interesting and stimulating Answers to the Question.

To Sajda Taha Mahmood:

Many thanks for your challenging Answer.

Do you mean the supposed 15 million registered academics on RG?

The exact number of academics registered in RG is: A0 = initial value (number of academics before counting new registration or dropout) e = exponential e = 2.71828183... k = continuous registration rate (also called constant of proportionality) (k > 0, the number is increasing (growing); k < 0, the number is decreasing (dropping out)) t = time that has passed. Regards- Rabiul

Interesting..

To Abdul Kuddus:

Many thanks for your illuminating and succinct one-word Answer.

To Rabiul Alam:

Many thanks for your algebraic equation to calculate the number of registered academics on RG, though we still need the initial condition, A0, which seems to be unknown.

How many academics are registered on RG who have died in the meantime?

To Romeo:

Starting from the time the Question was posted on 4 April 2019?

If we assume the attrition rate for academics on RG mirrors the world population, we could hazard a reasonable mean estimate, together with a standard deviation and higher moments.