Some 1 million of the networking site's 60 million users added the U.S. politics application to their personal pages, and a fraction of them have responded to polls or named their preferred candidate.
Their support for candidates, however, does not mirror the national opinion polls.
Paul has a following on the Web site unmatched in national polling. He is the favorite candidate of 37 percent of willing respondents who identify as Republicans. By contrast, Paul commands just 6 percent of Republican support nationally in many polls.
Former Gov. Mike Huckabee, who won Thursday's Iowa caucus, is supported by 17 percent of Facebook users versus 14 percent in a recent scientific ABC/Facebook poll.
"If people truly listen to Mike Huckabee tonight, they will see that he is not only a moral and honest person but is strong on all the issues," wrote Debi Large, a 52-year-old user from Okeechobee, Fla.
In an e-mail interview with ABC News, Large wrote, "I think Huckabee is doing great. He is honest, firm, calm and knows his stuff. I think Ron Paul comes across like a joke. McCain is doing well, Romney is so defensive and untrusting that even when he sounds good, I wouldn't trust him. Thompson never sounds trustworthy to me, and I'm tired of hearing about New York from Giuliani."
The Internet has already played a vital role in this year's campaign, with 40 percent of respondents to a scientific ABC/Facebook poll saying they go online for campaign news and information. Two-thirds of Americans say the information found online is important in deciding who to vote for.
One respondent who made his opinions known on Facebook in the lead up to tonight's debate was Democratic candidate Rep. Dennis Kucinich, D-Ohio, who was informed earlier that he would be barred from participating because he did not meet benchmarks for support.
ABC/Facebook "excluded me from the important New Hampshire debate, because I wasn't 'a Top 4 candidate' yet in the polls. Did you know Clinton/Obama/Edwards refuse to support impeachment, gay marriage and repeal of the Patriot Act and NAFTA? Or leave Iraq until 2013," he wrote in response to a Facebook poll, asking if electability was a factor in deciding one's vote in the primaries.
Republican candidate Rep. Duncan Hunter, R-Calif., and Democrat former Sen. Mike Gravel, D-Alaska, were also cut from the stage, given their poor showings in the Iowa caucuses and in New Hampshire and national surveys, as per ABC/Facebook rules.
On Facebook, Sen. Barrack Obama, D-Ill., is supported by 60 percent of Facebook visitors who voluntarily identified as Democrats and willingly noted their support of a candidate. By contrast, Obama is supported by just 20 percent of Democrats nationally, according to a scientific ABC/Facebook poll.