Chinese e-commerce platform Pinduoduo , which is known for its cut-to-the-bone price offerings through social media sharing, has quietly expanded its presence in Hong Kong with new delivery and payment services tailored for local consumers. The Shanghai-based company, which takes advantage of users’ social networks to promote deals and woo new buyers, has proven to be a formidable player in China’s “sink” market, referring to low-income consumers in rural areas. But as consumer spending in China has weakened, the platform, started by young Chinese billionaire Collin Huang in 2015, is now trying to test its services outside the mainland, including...