Twitter’s ad revenue growth will fall from the triple digits last year to “mere” 83% growth in 2012, eMarketer predicts, reaching nearly $260 million worldwide by the end of this year.

The social media service, which is continuing to roll out self-serve ad offerings, will see growth in ad revenues around the world taper off over the next three years yet remain solidly positive. By 2014, eMarketer estimates, Twitter will enjoy worldwide ad revenues of $540 million.

Currently, 90% of Twitter’s revenues come from US sources, with other countries contributing just $26 million to its ad revenues this year. The site will have diversified its revenue sources slightly by 2014, but 83% of dollars will still come from the US.

LinkedIn, which has lower revenues and a lower growth rate overall than Twitter, gets a greater share of its ad dollars from outside the US.

This year, when the site will see $226 million in ad revenues, a 46.1% increase over 2011, 32% of that money will come from abroad.

By 2014, US advertisers will account for 60% of LinkedIn’s revenues of $405.6 million, according to eMarketer estimates.

eMarketer forms its estimates of social network ad revenues through a meta-analysis of data from dozens of research firms, company information and industry trends. This forecast represents an upward revision of LinkedIn’s revenues and growth rate compared with eMarketer’s previous forecast, issued in September 2011, due to a stronger-than-expected advertising program at the site. Twitter’s ad revenues are close to those expected last September, with growth continuing on the expected trajectory for an additional year.