Chinese online game and social-networking service operator Tencent Holdings Ltd. said Thursday it will launch a new e-commerce platform next month, in a move that challenges more dominant rival Taobao Mall Co. Tencent is investing in areas including e-commerce to fuel growth outside of its key online-games business. Its new platform will compete with Alibaba Group Holding Ltd.’s Taobao, which dominates China’s online retail sector, and with a range of other companies in other e-commerce categories, such as group deals.