By Adam Jourdan SHANGHAI (Reuters) – Alibabas dominance of online retail in China faces its biggest-ever challenge as the firm founded by Jack Ma in a one-room apartment 15 years ago lines up a U.S. initial public offering that could value the firm at around $140 billion. In a rare blip, Alibaba Group Holdings lost market share last year while its nearest rivals all grew, according to Euromonitor. The market research firm sees Chinas internet retail market tripling from 2012 to over $300 billion in 2018 as the countrys smartphone-savvy shoppers buy everything from plane tickets to sneakers online. Chinas biggest social media company, Tencent Holdings Ltd, is leading the revolt, linking the countrys most popular messaging app, WeChat, with the number two e-commerce player, JD.com.