Twitter has generated more advertising revenue from its mobile platform than from its website on many days in the last quarter, CEO Dick Costolo said Wednesday, highlighting Twitter's progress in squeezing ad dollars out of the growing number of smartphone and tablet users worldwide.

Speaking at a conference hosted by The Economist Group in San Francisco, Costolo made his remarks at a time when newly public Facebook, a competitor in the social Internet arena, faces intense pressure to improve its business performance on mobile.

Facebook's stock has been battered since its troubled IPO in May, an event overshadowed by concerns that the social network -- first built in 2004 by CEO Mark Zuckerberg as a desktop Website -- would continue its struggle to monetize mobile users.

But Twitter, which is viewed in Silicon Valley as the most significant IPO prospect following Facebook, has taken pains to publicly distance itself from Facebook's struggles.

"We're borne of mobile," Costolo said in response to a moderator's question about the difference between Facebook and Twitter. "We have an ad platform that already is inherently suited to mobile, even though we launched our platform on the Web and only started running ads on mobile recently."

Twitter, which now has 140 million monthly active users producing 400 million tweets daily, has ramped up its revenue-generating efforts since Costolo assumed the top leadership role in late 2010.

The company introduced ads into smartphone users' timeline in February. The following month, Twitter released a feature allowing advertisers to send promotional tweets specifically to iPhone and Android users, who comprise 60 percent of all Twitter users, according to the company.

Twitter has closely held its revenue numbers, but digital media analysis firm eMarketer estimated in January that Twitter's revenues could reach $260 million in 2012 and $540 million in 2014.

Facebook reported $3.7 billion in revenue in 2011.