My view is that some advertisers are scared of controversial content, but the real problem is bigger than that. Clickthrough rates of banner advertising in these sites is low because of the mind state and task orientation of the visitor types - e.g. I'm here to catch up with my friends, not click on ads - and doing anything more elaborate for each site just isn't scalable.
It's one of the big challenges of social media. The future success of Facebook and other social media depends on a convincing answer.
Here is the link to the piece