By Anjali Athavaley

NEW YORK (Reuters) - Democratic presidential candidate Bernie Sanders dominated overall conversation about the Iowa caucuses on Facebook Inc on Monday, the social network said.

From midnight to noon CST (0600-1800 GMT), 42.2 percent of conversations about the caucuses was about the senator from Vermont, compared with 21.7 percent for Republican front runner Donald Trump and 13.1 percent for Democrat Hillary Clinton, according to Facebook.

The caucuses held on Monday night are where the first votes are cast for the U.S. presidential nominations and where Clinton is locked in a tight race with Sanders to become the Democratic nominee for the November election.

The Facebook data is surprising given Trump's success in using social media as a campaigning tool in his presidential bid. The real estate tycoon has been particularly active on Twitter Inc , with more followers and tweets than any other candidate running for president.

Social media posts do not necessarily translate into votes, but experts in digital strategy say they can indicate levels of enthusiasm among active supporters.

Senator Ted Cruz from Texas, Senator Rand Paul from Kentucky and retired neurosurgeon Ben Carson, all candidates in the Republican race, respectively accounted for 10.7 percent, 4.7 percent and 2.6 percent of Facebook conversations about the caucuses.

The top three issues discussed were the economy, same-sex marriage and State Department emails, the social network said.

The U.S. State Department conceded for the first time on Friday that intelligence officials were correct to say that at least 22 emails sent through Hillary Clinton's private server contain some of the government's most sensitive secrets.

(Reporting by Anjali Athavaley; Editing by Marguerita Choy)