The growth in ad spend on social networks may have triggered a drop in the use of those networks, reports BusinessWeek.

The average length of a user's visit to social networking sites fell 14 percent in the last four months, said comScore. And while userbases continue to grow, they do so at a slower rate than in the past.

Contrast that with a 115 percent increase in money spent advertising on those sites last year. Ad spend on social networking is expected to grow 75 percent this year.

Google recently admitted to experiencing difficulty monetizing search ads on MySpace. And if not for its guaranteed payment agreement with Google, Fox Interactive would be making little, if any, profit on the popular site.