HONG KONG—Colin Huang, founder of the Chinese e-commerce firm Pinduoduo, stepped down as chairman Wednesday just as the company’s annual user base surpassed that of rival Alibaba. Huang, who founded Pinduoduo in 2015 and is China’s third-richest man with a net worth of about $56 billion, said that he is giving up the chairmanship to pursue research in food and life sciences. Huang’s board seat will be taken by CEO Chen Lei, according to the company. Shares of Pinduoduo, which is listed on Nasdaq, tumbled 7 percent following the announcement. Huang departs with Pinduoduo achieving record growth. The company on Wednesday said its annual active users reached 788.4 million in December, up 35 percent compared with the same time last year, and exceeding the user base of 779 million counted by rival Alibaba Group Holding. Quarterly revenue spiked 146 percent from a year earlier, to $4 billion. For comparison, in …