Unless Twitter can come with a new fresh ideas to make their user more engage and hook to their services than some other service may take the lead (i.e. Tumblr, FriendFeed even Microsoft thorough their OfficeTalk)

Facebook as we know already become the numero uno of social networking sites. It actually more like a virtual country (zuckerberg want to create virtual economy through virtual credit) with population is 3rd largest in the world.