Based on the slowing rate of membership growth, I decided to fit an exponential rise curve to see how well it fits, and what it might predict.
The curve that best fits the data is one that has the formula
y = 6024 * (1 - exp( -.1391 * x ) ) where x is months since introduction.
This is a curve that asymptotically approaches an upper limit of 6024, and reaches 6000 at roughly 36 months.
Plotting the actual values on the curve shows a relatively good fit.