According to comScore, the average number of minutes spent on Facebook among 18- to 24-year-olds fell in September for the third consecutive month compared to the same period a year ago.  In July 2009, usage fell 3 percent, in August 13 percent and in September 16 percent.  Separately, Mindshare  surveyed 1,200 consumers about their social-networking habits in August 2009.  More than half of the 18- to 24-year-old respondents (51 percent) agreed that “social-networking sites like Facebook are diluting the quality of relationships.” 40 percent of that group said they now visit social networks that are based on particular interests, such as TV, music or movies.  More analysis at Adweek.