San Francisco: Facebook plans to announce on Monday that more than 5 million businesses are advertising on the social network each month and that it is updating its suite of ad services to try to draw more small businesses onto its mobile platform.

Chief Operating Officer Sheryl Sandberg said in an interview 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.

Facebook is locked in a battle with Alphabet Inc's Google for dominance in digital advertising. The two Silicon Valley giants are expected to soak up some 46 per cent of online ad spending in 2018, 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.

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 5 million advertisers, the biggest industries are e-commerce, entertainment/media and retail, the company said. Some 75 per cent of them are outside the United States, and its fastest-growing markets are India, Thailand, Brazil, Mexico and Argentina, the company said.

Nearly 50 per cent of the advertisers create ads on mobile devices, reflecting the increasing importance of smartphone-based usage to Facebook's future.

More than 90 per cent of Facebook's users access the network through mobile devices, and mobile advertising accounts for 84 per cent of ad revenue, the company said in February.

Reuters