After selling shares in a highly anticipated initial public offering, Twitter is set to face minute ongoing scrutiny from a legion of new shareholders and Wall Street investment analysts. Waning appeal in parts of Asia, once the fastest growing market for the social network that revolves around users posting 140-character messages online, shows the challenge Twitter faces in keeping overseas users engaged. Celebrities, pop stars, politicians and opinion leaders flocked to Twitter, turning an experimental playground for early adopters into a dynamic online forum. New mobile applications from companies such as South Korea's Kakao Corp.