• by Joe Mandese  @mp_joemandese, August 30, 2012

Citing  its under-performance during the first half of 2012, eMarketer has shaved $1 billion off its previous projection for Facebook's revenues this year. In February, eMarketer predicted Facebook's revenues would top $6 billion this year, but not projects it will grow to only $5.042 bilion, a downward revision of 17%.

The new 2012 estimate nonetheless represents a 35.9% increase over the $3.711 billion eMarketer estimates Facebook took in 2012 revenues.

eMarketer also estimated that advertiisng currently represents 83.9% of Facebook's revenue sources, which declined slightly from 85.0% in 2011.

"In 2013, revenue is expected to increase 31% as Facebook fully rolls out new ad products such as its ad exchange. International revenue is also expected to climb with the expansion of the company's mobile ad business," the eMarketer report notes, adding that it's downward revision for 2012 revenue projections "does not reflect concerns about Facebook's mobile advertising business. While the February forecast included the assumption that Facebook would start to sell mobile advertising this year, eMarketer did not expect that it would become a meaningful contributor to revenue in 2012."