The number of 15-24 year olds in the U.K. visiting social networking sites reached 6.8 million in June, up 14 percent compared to the previous year, according to a new report from comScore.

The overall audience of social networkers in this age group is up substantially during the past year, but the time spent on social networking sites is down 9 percent. The overall decrease in time spent on the sites appears to be due to younger users spending less time on secondary social networking sites.

"Recent reports have suggested that 15-24 year olds in the U.K. are moving away from social networking sites because they’re no longer cool as older users have encroached on their virtual space. comScore research indicates that is simply not the case," said Mike Read, SVP and Managing Director, comScore Europe.

"In fact, more 15-24 year olds are using social networking sites than a year ago, so there isn’t any particular aversion to the activity. What does appear to be happening is that younger users are beginning to consolidate around Facebook and are spending less time on competing sites."

Younger users are increasingly moving towards Facebook as their main social networking destination, even though Facebook has a significantly older user profile than MySpace and Bebo. Specifically, 77 percent of Facebook visitors are age 25 or older, compared to 65 percent of Bebo visitors and 69 percent of MySpace visitors.

"These data suggest that not only are younger social networkers not avoiding their older counterparts, but they are actually gravitating towards a site that skews more heavily towards users age 25 and older," added Read.

"It’s likely that the age of one’s peers on a social networking site is not a critical factor in how 15-24 year olds will spend their time online. But as people’s digital media lives become increasingly fragmented, users are craving the simplicity of fewer platforms – and the dominant social networking platform right now appears to be Facebook."

Research from Nielsen shows that time spent on Facebook is up 700% from a year ago (that’s April ‘08 to April ‘09). Meanwhile, MySpace has experienced -31% year-over-year growth.

That doesn’t look great for MySpace, but don’t count out the still popular social network. MySpace is still beating Facebook when it comes to video. In April, Myspace visitors spent 384 million minutes watching video (an average of 38.8 minutes per viewer), while Facebook visitors spent only 113.5 million minutes watching video (an average of 11.2 minutes per video viewer).

"We have seen some very exciting growth in Facebook during the past year, and a subsequent decline in MySpace. Twitter has come on the scene in an explosive way perhaps changing the outlook for the entire space. The one thing that is clear about social networking is that regardless of how fast a site is growing, or a how big it is, it can quickly fall out of favor with consumers," said Jon Gibs, vice president, media and agency insights, Nielsen Online. "Remember Friendster? Remember when MySpace was an unbeatable force? Neither Facebook nor Twitter are immune. Consumers have shown that they are willing to pick up their networks and move them to another platform, seemingly at a moment’s notice."

Gibs makes a very valid point. Look at the backlash that Facebook got from its users after it implemented the infamous redesign. Granted, the redesign is still in effect, and the site is still growing, but if something else comes along that people like better, you never know what can happen. In fact, this is why I find Google’s rise as a social network all the more intriguing.

But for now, Facebooks is obviously the dominant force to be reckoned with. It does have a problem with user retention from the over-55 crowd. Justin Smith suggests the following four reasons for this:

1. Facebook currently provides less value to this age group because most of their friends are still using other communication tools like email.

2. The real-time Facebook stream is too new and overwhelming for people less familiar with social networks to understand.

3. Seasonality of family communication.

4. Facebook was just a one-time thing.

Smith goes into detail to back up each of these reasons in an interesting post here. He also looks at audience growth in the last two 60-day periods, providing the following graph:

As you can see there has been substantial growth among the college age crowd. A while back, a study was released, indicating that Facebook was actually having a negative impact on the grades of students. The study looked at Ohio State University specifically.

The Wall Street journal is showing a little more to this story now, citing newer studies that actually show Facebook use as being more common with students who get higher grades. Like anything else, it’s how you use it, and not if you use it.

What do you make of the Facebook growth data? How about Facebook’s correlation with grades? Talk about it.