Search grew rapidly as more people got online looking for information. Smartphones also boosted usage and revenue climbed after Google loaded more ads into the top of mobile search results. But there are limits to the growth of such a large business.

Google can only stuff so many ads onto its website without lowering the quality of search results. On mobile phones, ads often fill the entire screen, forcing users to scroll down if they want to see free listings. Over the years, Google has used various tweaks to wring more clicks out of search ads. But there may be limits to that, too. Recently, it changed the way ads are labeled, causing some people to say it was trying to blur the line between ads and free results. The company quickly backtracked.

Google chief Sundar Pichai. Credit:Bloomberg

And fewer people are joining the internet in the most lucrative search ad markets. From 2017 to 2019, the number of internet users in Europe grew 10 per cent, while in North America growth was only 2 per cent. In Asia, the online population jumped 19 per cent, according to data aggregation company Statista. Google worked on a censored search service for China, the world's largest internet market, but scrapped the project after some employees and US politicians criticised the effort.

Some of the most valuable search ads - those for specific products that people can buy - face competition, especially from Amazon.com. About half of product searches start on Amazon now, Bernstein's Shmulik wrote in a research note earlier this year. That's spurred Google to build new kinds of shopping ads, but it hasn't reversed the growth slowdown. Searches for decorating ideas and clothing are also increasingly happening on social networks such as Pinterest and Instagram. And millions of people look for music on Spotify's mobile app, not Google.