Which of these is the top Twitter city in the US?
You have your ``Top Twitterers,'' users with the most followers, even most influential ... but can you guess which of these is the top Twitter city in the US?
Twitter, the world's most popular microblogging site, is already making money and must become even simpler and more unified across platforms in the future, CEO Dick Costolo told an audience at the Mobile World Congress in Barcelona Monday.