Facebook's chief operating officer, Sheryl Sandberg, has admitted that the world's biggest social media company made a string of blunders in underestimating the impact of mobile, but said usage on smartphones and tablets was now taking off and was "even bigger than we thought".
Ms Sandberg told the Cannes Lions advertising festival that Facebook made "a pretty big mistake" because it did not tailor its site and advertisements for smartphones and tablets until 2012.
"We weren't just late. We made a wrong bet," she added, explaining that Facebook's decision to build a single mobile app that worked across all devices, using a universal computer language called HTML5, had backfired.
Facebook has had much more success with "native" apps tailored for each operating system, such as Apple's iOS and Google's Android, and is now getting 59 per cent of ad revenues from mobile against virtually nothing two years ago.
"Mobile is even bigger than we thought," said Ms Sandberg, who cited research showing that people spend one in every seven minutes online on Facebook, but that rises to one in every five minutes on mobile devices.
She promised further improvements to Facebook's news feed, the main part of the site where people see posts from friends and advertisers. "It could still be better," she said.
"We could be showing you more things you want to see. You can have some friends who post annoying things. We haven't got to the stage where every ad on Facebook is a delightful experience, but we can get there."
Some advertisers have voiced fears in recent months about how Facebook has already changed its algorithm, making harder for brands to reach large audiences, and is also driving up prices.
Ms Sandberg pointed out that the wider advertising industry was being slow to shift budgets to mobile, noting that about 20 per cent of consumers' time was spent on mobile devices ? but only 4 per cent of ad budgets.
She joked how Huffington Post's Arianna Huffington had recently said 20 per cent of people looked at their smartphones during sex because they were so addicted to their mobiles.
The mobile boom has helped Facebook's shares to power ahead, after they initially fell at the time of its $100bn float in 2012 amid fears of slowing growth because it was lagging in mobile.
Ms Sandberg said some observers had underestimated founder Mark Zuckerberg's abilities as a chief executive of a publicly listed company "mainly because he's shy". She added: "He's one of the best listeners in the world. He's an optimist and believes in the power of individuals."