I would say that the 81% figure is very typical for websites of this genre. I have a reasonably sized website for graphic artists and photographers with about 133k users over the past 3-4 years. I also see exactly 81% 'dead' users (accounts not used in the last 30 days).

I would say that the percentage of active users is going to mainly be dependant on the time that the site has been opperational as well as the site genre (to a lesser extent).