I'm getting sick and tired of playing this round-robin game with social networking every couple of years moving everything from Myspace to Facebook and now possibly Diaspora
Gotta go with what works and what suits you personally. While if in it for the self promotion, its not bad having an account with every network out there, Just find a spot and live there.
Myspace was OK, it became a group of poorly coded webpages and a dump of youtube poop videos and bad pirated music.
Facebook seem more formal, to the point, your life on the web, yet offers too much fodder for stalkers
Twitter pretty much put the status update from facebook, and made it into a giant message board. Sometimes i link it as a hybrid IRC chat room / message board postings with no rooms, no subjects.
Google's lattitude tried to clone Facebook but was pretty much a merger of all the other google apps and added "status update". Plus abilities to make yourself super stalkable by having yourself placed on GoogleMaps via realtime for a "Look! I'm HERE!!" feature.