The market research company has estimated that Twitter will earn $129.7m (£81.3m) in mobile advertising revenues this year in the US with Facebook coming in at just under half that at $72.7m.
Facebook and Twitter earn far less in the US than established mobile ad platforms, such as Google, which saw revenues of $750m from mobile in the US last year.
Emarketer estimates it will almost double that this year, making $1.42bn in mobile advertising, with a market share of 54.4%.
Both Facebook and Twitter have publicly said their future business development lies in mobile. According to eMarketer, the tight integration between Twitter’s ad products, and in particular Promoted Tweets, and its core user experience, has made the transition to mobile "relatively simple".
In July, Dick Costolo, chief executive of Twitter said that on some days it made more revenue from mobile than desktop ads.
Facebook rolled out mobile advertising in newsfeeds earlier this year. It had identified its mobile users' lower exposure to advertising and the fact that nearly half of its users are accessing the network from mobile devices as a potential risk factor in its IPO prospectus.
While it is yet to crack mobile advertising it has been ramping the up its commercialisation in recent months.
According to eMarketer Facebook makes an estimated 60% of its revenue from its Marketplace advertising platform, most of which is generated by ads that display on the right-hand side of Facebook's desktop website. These ads do not appear on its mobile platform.
In the longer term eMarketer expects Facebook to overtake Twitter in mobile advertising revenues in the US. By 2014, it forecasts Facebook’s US mobile revenues will reach $629.4m, compared to $444.1m by Twitter.
It has not revealed predictions for the UK mobile advertising market, which is smaller than the US.Follow @shearmans
This article was first published on mediaweek.co.uk