A couple of years ago, Bryce Glass of Soldier Ant blog drew up a fantastic graph called the Flickr User Model, explaining exactly what they can do with in the Flickr-verse.
Link [Flickr, of course. Larger size] - via Infovis
Graphoholics be ware: Infovis has an incredible searchable database of 1,200+ examples of graphs. You'll lose hours there if you're not careful.