Writing in Ad Age, Steve Rubel shares the following trope… (thanks to my favorite ads-keteer for the link):

Galaxies: centers of gravity pulling together like-minded individuals, such as YouTube, Digg, Flickr and Second Life.

Stars: online celebs, such as Microsoft blogger Robert Scoble, Flickr fiend Thomas Hawk and YouTube addict Nornna.

Planets: individuals who follow the stars, influential in their own right.

Shooting stars: insta-celebs that create neat videos and then fade away.

Comets: recurring themes, such as transparency, veracity and entitlement.

Asteroids: desolate, lifeless places with negative energy — think spam blogs.

(Yes, Rubel’s column has an RSS feed.)