Editor's note: Amy Henry is vice president, youth insights, of C&R Research, Chicago.

For all the curiosity surrounding the number of Baby Boomers flocking to Facebook, social networking is still a young person's game. Membership among teens age 13 (technically Facebook's age limit) to 17 grew an impressive 90 percent from 2009 to 2010, with 257,000 joining in June 2010.

This shouldn't come as any surprise. Both Facebook and its closest competitor, MySpace, are ideal to meet the needs of older tweens and teens as they become increasingly focused on learning the complex rules of friendship, and especially how friendship plays out in today's world of online connections and virtual relationships.

But marketers, beware. Courting the under-18 crowd using traditional tactics has always been tricky. Those intending to campaign on this turf for their hearts and minds must step carefully and stay true to the unwritten ground rules of authenticity and relevance. A brand's success with youth via social media channels hinges on establishing a presence that delivers a sense of community that tweens and teens can then use to tell the rest of the world that they "belong" to the brand as much as the brand belongs to them.

The friends they keep

It helps to first understand where the 13-to-17-year-old crowd stands developmentally. While they understand that the friends they keep say something about who they are, they may be less aware that it's perfectly normal for youth in this group to  "try on" different types of friends.

But this behavior plays a powerful role in teens' search for identity. By associating themselves with different kinds of friends, they not only get to see what it feels like to play different roles within a group, but they also get to see how it feels to assume different personas. Wondering where they fit and to whom they belong makes social status and standing a borderline obsession for most tweens and teens.

Social networking communities like Facebook and MySpace remove much of the guesswork for crowd-conscious youth. Through these sites, tweens and teens can connect with friends at all hours, strategize how to present themselves or respond to posts and broadcast to groups of friends at once. They also facilitate self-expression by giving them the chance to affiliate with groups or support brands by "liking" them with a simple click of the mouse. And, of course, for this quant-driven generation, the ability to count friends and show off how many they have is one way to feel loved.

Walk a fine line

To effectively command their attention while they're on these sites - much less forge a relationship with them - marketers must walk a fine line. Youths actually think of brands as friends, and if they love your brand, then you'd better have a social media presence for them to feel it's relevant. By the same token, however, your brand should not infiltrate or intrude on their space. So your brand's presence in these communities should be less about promoting a product or service and more about providing expertise or information to them that feels of value to them, not to you.

While North Face's market spans a much broader demographic than tweens and teens, its social media efforts provide a model worth exploring. North Face tweets about hiking-, running- and skiing-oriented events. Its Facebook page encourages fans to discuss "gear, athletes, sports, trips, tips, comments and questions" - some of which links directly to products for sale, but much of which is about building a sense of community around a lifestyle. And not to be forgotten, its apps help smartphone users check on ski conditions before they hit the slopes.

Rules for building brand trust

Using these channels to reach the older youth market can be rewarding. However, it's especially important for marketers to understand that the rules for building brand trust with a tween or teen via a social network are different than in the offline world. And because of the growing number of youth participating in social networks, it's essential that marketers build their social media strategies on a foundation of authentic consumer needs.