Twitter is a pulse, and people have a tendency to follow those trends. In the case of FB, it attracted tonnes and is maintaining that growth due to the way it works - it is multi-feed.
Twitter offers 'snippets' for busy people. I don't see the trend for Twitter lasting, as +David De Silva
mentioned - G+, FB, and others are catching up with the 'one liner' and 'news feed' style. As for MySpace...i even think i still got an account along with Windows Live Messenger!?