NEW YORK/BEIJING (Reuters) – Twitter Inc executive Kathy Chen, brought in to run Greater China just over eight months ago, has quit, according to a tweet sent by her over the weekend. Twitter has been blocked in China since 2009 but is still used through virtual private networks (VPN). "Now that the Twitter APAC team is working directly with Chinese advertisers, this is the right time for me to leave the company," she wrote.