Chief Operating Officer Sheryl Sandberg said in an interview with Reuters that the number of ad buyers was an important milepost, showing that Facebook has room to grow despite its massive scale.
The company said in September that it had 4 million advertisers, and in March 2016 that it had 3 million. The world's largest social network, which is free to users, has 1.9 billion people on it.
Among the 5 million advertisers, the biggest industries are e-commerce, entertainment/media and retail, the company said. Some 75 percent of them are outside the United States, and its fastest-growing markets are India, Thailand, Brazil, Mexico and Argentina, the company said.
Nearly 50 percent of the advertisers create ads on mobile devices, reflecting the increasing importance of smartphone-based usage to Facebook's future.
More than 90 percent of Facebook's users access the network through mobile devices, and mobile advertising accounts for 84 percent of ad revenue, the company said in February."