Market research company eMarketer has raised its forecast for advertising revenue on Twitter, with the firm predicting $582.8 million in ad spending on Twitter in 2013 and $950 million in 2014.

This is an upward revision from a previous report that forecast $800 million in ad revenue for Twitter by 2014.

The company expects advertising on mobile devices to be the area where Twitter sees the biggest growth over the next two years, with more than 60 percent of an estimated $1.33 billion in advertising income expected to be delivered via mobile sources by 2015.

Indeed, Twitter’s strong presence in the mobile space, largely thanks to the native nature of its ad products, is tipped to play a key role in the micro-blogging social network’s continuing growth, with $308.9 million in mobile ad revenue targeted for this year, rising to $551 million in 2014 and $811.3 million in 2015.

While the bulk of Twitter advertising revenue remains in the U.S., with just 17 percent of the worldwide total coming from non-U.S. sources, this figure is also expected to rise over the next two years, with international advertising revenue from international sources reaching almost one-quarter (24 percent) of total advertising income by 2015.

(Source: eMarketer.)