webmaster forums like dp and warez, most of the forums i have seen are warez and Webmaster, in warez people go for premium signups if you have good content
Well we all knows that Porn is very popular niche and very active too beside of Money Making/Webmaster forums. I know a Porn forum where you will find atleast 5000 people Online at any time.
Yes sure, It is exbii.com which always be active and you will find such huge members at any time. Now go their and make 5001, don't stay for long time.