Facebook gathered figures on the number of fans each GOP candidate had on the site between Dec. 15 and Jan. 14, Politico reported Tuesday.
The data was gathered from the candidates' official Facebook fan pages.
Texas Congressman Ron Paul, known for making use of sites like Facebook and Twitter to advance his campaign, was second with about 800,000 Facebook fans.
The rest of the GOP pack was far behind, with all of the candidates clustered around the 200,000 mark.
Romney's rise on Facebook began shortly after Christmas and continued through the Iowa caucuses, when more than 100,000 people were sharing information about the former Massachusetts governor's page.
It peaked last Wednesday, the day after he won the New Hampshire primary by a large margin.
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