Any person under 30 does what? Keeps tabs? Anonymously? Or posts? And, do they use their REAL names?
In my experience, most younger people keep tabs anonymously on family, friends and acquaintances. MySpace and Facebook has made it easy and normal for most younger people to be stalkers.
And addressing your question from your vanity, most people do use their real names in the searchable profiles. And also most teenagers who use a computer have one. You should probably be concerned if they have shifty friends on there or are dressed inappropriately, but most use it just to keep in touch with their friends. Myspace became popular in my early 20s and I use it primarily to meet people and "stalk" friends. Many Freepers also gather there to join conservative groups, unbenownst to older non Myspace freepers.