Chief operating officer Sheryl Sandberg said the number of ad buyers was an important milepost, showing that Facebook has room to grow despite its massive scale.
The company said last September that it had four million advertisers, and in March of last year that it had three million. The world's largest social network, which is free to users, has 1.9 billion people on it.
Facebook is locked in a battle with Alphabet's Google for dominance in digital advertising. The two Silicon Valley giants are expected to soak up some 46 percent of online ad spending next year, according to research firm eMarketer.
Much of that spending is by big brand advertisers, but to attract small firms, Facebook pitches a set of tools such as audience targeting, a mobile design studio and online courses available so far in 10 languages.
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The latest changes include a “one click” option in Facebook’s ads manager to reinstate high-performing ads, and a desktop tool to manage customer messages, the company said.
“Small businesses provide such important products and services, but they also are a huge engine of job growth in a world where job growth is increasingly uncertain and something people really worry about,” Sandberg said.
Among the five million advertisers, the biggest industries are e-commerce, entertainment/media and retail, the company said. Some 75 percent of them are outside the US, 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.