The xkcd webcomic has a new edition out of its Map of Online Communities. Go take a look (the map enlarges if you click on it).
What the map does well is to place otherwise unconnected Internet communities into perspective. For example, political bloggers who suppose the wired world reads their every flame may be chastened to find the map needs an enlarged inset to show them. In some regions one group dominates - Warcraft on MMO island, Skype in messaging, and Facebook as the biggest nation.