It seems Facebook's planetary ubiquity is inevitable ... until you take a look at the sheer size ofthose competitors: Total membership of the four major non-U.S. networks total 778 million. Combine that with Twitter's 500 million active users around the globe, and you've got a social horde that eclipses Facebook's. Besides Twitter (whom you've no doubt met), who are these plucky social networks standing firm outside the U.S.?