The boss of social network Facebook has spoken for the first time of the drop in his company's market value.
Mark Zuckerberg called the drop in his firm's value "disappointing". The value of its shares is almost half the $38 debut price in May.
But he vowed that Facebook will make more money on phones than on desktops.
"Over the next three to five years, the biggest question on everyone's mind is really going to be how well Facebook does with mobile."
Speaking at the TechCrunch Disrupt conference in San Francisco, Mr Zuckerberg said: "Literally, six months ago we didn't have an ad on mobile."
Earlier this year, Facebook launched native apps for Apple's iOS smartphones.
"Ads have to be more integrated into the product on mobile," Mr Zuckerberg said.
In another exchange, he joked: "Everything I do breaks, but I fix it quickly."
'Moving the needle'
Facebook is the world's most popular social network with 950 million users.
When asked of constant rumours that he was building a Facebook phone, he rejected the speculation and pointed to the site's huge reach.
"If we make a phone we could get maybe 10 million users? Twelve million users? That doesn't move the needle for us.
"Building a phone is the wrong strategy for us."
He admitted the fall in Facebook's share price had made it harder to find and retain staff. "It doesn't help," he said.
"There are tons of people that are super-pessimistic," Mr Zuckerberg said. "I would personally rather be underestimated. It gives us latitude to go out and make some big bets."
Since their debut at $38 in May, Facebook shares have lost 49% of their value. They closed at $19.43 on Tuesday.
Mr Zuckerberg owns about 444 million Facebook shares plus an option to issue another 60 million.
Last month, Peter Thiel, a venture capitalist and one of Facebook's earliest backers, sold much of his stake and made more than $1bn in total from his investment in Facebook.