Twitter is on track to earn almost twice as much U.S. mobile advertising revenue than Facebook in 2012, says a new forecast from eMarketer.

eMarketer projects $129.7 million in mobile ad revenue for Twitter in 2012, with Facebook pegged at just over half that amount at $72.7 million.

Both companies are dwarfed by Google, which is expected to rake in $1.42 billion from its mobile advertising platform this year.

Note that Twitter’s victory will be fleeting, with Facebook’s massive userbase expected to push its mobile ad revenue well above that of Twitter’s in 2013 and 2014, where it is envisaged to become the second biggest player in the market, commanding a 9.5 percent share.

Google’s overall share of more than half the mobile advertising space is expected to remain constant through 2014.

(Source: eMarketer.)