Ning is going to be reaching a big milestone this week, when it sees its one millionth social network created on its self-serve platform. Ning, which had its initial beta launch in 2005 and ‘full’ launch in 2007, makes it easy for people to create niche social networks focused on the topics they care about.

Ning continues to grow steadily despite the fact that it banned ‘adult’ social networks late last year (some had suggested that the site relied on these adult networks to drive a significant amount of traffic, which isn’t the case). Ning now reports 22 million registered users overall, 700,000 of whom have created their own networks. Of the 1 million networks created, 200,000 remain active, across which 2.5 million new pieces of content (including comments and photos, and other media) are added per day.

Recently the site has been adding a number of new features that help its network creators spruce up their networks, including a persistent chat feature similar to Facebook Chat. The site also recently revamped its homepage at Ning.com to make it easier for users to manage their activity on multiple social networks and to discover new ones. Network Creators can also access a new directory of OpenSocial applications which run the gamut from games and Ecards to enhanced messaging services.

Because many of its users (particularly network creators) tend to be quite passionate about the sites they’ve built, Ning hasn’t been without its share of controversy, but its continued growth seems to indicate that most users are satisfied.