-By Steve McClellan

Just a few years ago, social networking site Facebook became an almost overnight sensation, propelled by a college-age crowd that saw the site as an exclusive sanctuary to connect with peers.

Now it seems, as Facebook has gained a broader audience, the older teens and twentysomethings that drove its initial popularity are using it less, according to comScore.

And according to research by WPP’s Mindshare, that group is re-evaluating its worth. Others agree that Facebook’s cool factor with younger users appears to be waning. “When you start getting friended by your grandmother, I think that’s when it starts to lose its cool,” said Huw Griffiths, executive vp, global director of marketing accountability and research at Interpublic Group’s Universal McCann.

The lower numbers have fueled a debate among agencies about the implications for marketers. For some, it has raised a warning flag; if the trend continues, clients may have to revise their social-network marketing strategies. Others said Facebook’s broader growth outweighs declining usage by young adults. And still others said they may just be accessing the site via mobile devices that don’t show up in the numbers. Facebook execs declined to comment.

According to comScore, the average number of minutes spent online at the site by people 18-24 fell for the third consecutive month in September compared to the same period a year ago. In July, Facebook usage fell 3 percent, in August 13 percent and September 16 percent.

Mark Potts, North American managing director for consumer insights at Mindshare,  said signs of change began appearing a year ago when members of the shop’s so-called “Scout Network,” a group of trendspotting consumers across the country, began reporting shifts in the way its friends were using the site.

But others aren’t so sure. “That [usage decline] could be for a small percentage of the age group…I would want to see more evidence to show that that audience is running away from Facebook,” said James Kiernan, senior vp, group client director, MediaVest USA.