The ban on the popular social networking site is the latest crackdown on dissent in Tajikistan a year before an election that could extend Rakhmon's two-decade rule.
Beg Zukhurov, head of the state-run communications service that is enforcing the ban - the second time Tajikistan has blocked Facebook this year - accused unnamed donors of paying users to post negative comments about "respected figures".
"The best representatives of the public - among them academics, doctors and important cultural figures - are tired of the stream of mud and slander that flows from the website called Facebook," Zukhurov told Reuters by telephone on Tuesday.
"With this public support, a decision was taken to block this site, where some people are receiving $5,000 to $10,000 for every critical comment that they post."
He did not offer any evidence for this allegation or say who might be funding these posts.
Rakhmon has ruled since 1992 in Tajikistan, an impoverished ex-Soviet republic of 7.5 million people lying on a major transit route for Afghan drugs to Europe and Russia.
Victory in a November 2013 election would give the 60-year-old former cotton farm boss a further seven years in charge of a country still finding its way after a civil war in the 1990s that killed tens of thousands.
In recent months, the government has turned its attention to damping down dissent by creating a volunteer-run body to monitor Internet use and reprimand those who openly criticise the government.
WARY OF SOCIAL MEDIA
Tighter Internet controls echo measures taken by other former Soviet republics in Central Asia, where authoritarian rulers are wary of the role social media played in revolutions in the Arab world and mass protests in Russia.
Tajikistan authorities have also launched a crackdown on religious groups and imprisoned more than 150 people in the last three years on charges of extremism and attempting to subvert the constitution.
Officials have blocked access to Facebook before, for the same reason. The site was shut for 10 days in March, prompting criticism from the Organisation for Security and Cooperation in Europe.
Facebook's popularity has soared in Tajikistan. Membership has trebled to more than 40,000 over the last 18 months.
"Does Facebook have an owner? I'd like to speak to him," Zukhurov said. "Let him come here and meet me in my office."
Zafar Abdullayev, a political analyst in the capital Dushanbe, said he believed the ban reflected concerns about rising public criticism ahead of the presidential election.
"We can expect to see more steps to restrict freedom of speech on the Internet, as the authorities have made no secret of the fact they see a real threat in social networking sites," he said.
The website of Russian news agency RIA Novosti and state-run television channel Rossiya-24 were also blocked along with Facebook in March, and access has not been restored.
Some Internet users in Tajikistan, however, have installed software that allows them to circumvent the blockage.
© Thomson Reuters 2012