I don't like to make too big of a deal about registered user numbers — there are just too many factors involved for something like that to be a real indicator of an online community's success — but 300,000 is a very cool milestone. To have that many people take time to create an account on the site is an amazing complement to the quality of our community here, and I must thank each and everyone one of you that has done so. I still remember how excited I was when we got our first 1,000 members; I never imagined we would one day get that many registrations in just a few days!
Around 20% of registered users are active, with roughly 60,000 people signing into their GTPlanet account each month (of course, that goes way up around big game releases). I don't know how that compares with other forums, but that ratio has held steady for the last decade or so and is in line with the large social networks (FB, Twitter, etc.) which have 20-30% active user bases.