According to Nielsen-NetRatings, social networking sites (MySpace, Friendster, etc.) grew by 50 percent last year.  And they are still growing at a very rapid rate.  Advertisers are scrambling to take advantage of this audience, but one thing stands in the way - inappropriate material.  Understandably, Walmart doesn't want their ad placed next to a nude photo or an explicit blog post. 

The Washington Post recently featured a story about social networking site Tribe.net detailing how they lost members when they started censoring or removing material - paving the way for advertisers.  It's a fine line between offering a "free-speech" style site and restricting content so that the site becomes everything that social networkers are trying to avoid.

While Tribe.net claims that the lost members didn't hurt their overall business, it's worth paying attention.  When designing your site, it makes sense to plan ahead for potential advertisers.  It's better to lay the rules out for your new network, blog or message board in the beginning, than to have to change them after the community is built and risk total desertion.