from wsj.com: For some advertisers, 218 million people isn't a big enough audience. That's their message to Twitter Inc., which Thursday detailed plans for an initial public offering. Ad buyers say that the short-message service will need significantly more users, and a bigger sales force, to win more spending from their mass-market clients. "Scale still matters," says Adam Shlachter, senior vice president of media at DigitasLBi, a digital-ad firm owned by Publicis Groupe SA. "How consumers embrace [Twitter] and tap into it or tune in or out is going to be critical."