Social networks get a lot of praise from marketers looking to make sense of an increasingly fragmented media landscape. But a recent BusinessWeek story suggests that users on the sites have become immune to advertising.

While some users have reported anecdotally that they are no longer responsive to ads on sites like MySpace and Facebook, clickthrough rates have borne out this assertion. According to numbers reported in BusinessWeek, average clickthrough rates on social networks have dropped from one in 100 in 2006 to one in 1,000 through 2007.

A recent Wall Street Journal article also highlighted the difficulty in advertising on social networks. According to financial reports from Google, the search giant has had trouble growing its advertising business with respect to monetizing social networks and online video. By contrast, Microsoft, which took a stake in Facebook, contends that it has seen steady growth in the social networking space.