Twitter’s share of advertising revenues in the U.S., which was entirely desktop-based as recently as 2011, is now mostly mobile, suggests a new report from eMarketer.

Mobile ads are expected to account for 55 percent of Twitter’s ad revenue share in 2013, up from 52 percent last year, and zero in 2011. Additionally, further growth in this space is also envisaged, reaching 62 percent by 2015.

Conversely, Facebook’s ad revenue share remains predominately desktop-based this year, but is also expected to switch to mobile in 2014.

(Source: eMarketer.)