Former opposition leader Sam Rainsy's legal team filed the lawsuit in California last week, saying that Hun Sen was using the platform to commit human rights abuses and deceive the electorate.
Among other allegations was that Hun Sen was paying for false Facebook "likes" to mislead voters about his support.
"This person is crazy and stupid," Hun Sen told thousands of university graduates in the capital Phnom Penh, saying Rainsy was just jealous because his official Facebook page had 9.4 million "likes" to only 4.5 million for Sam Rainsy's.
Hun Sen said he had no idea who had liked his own page.
The case has drawn attention to the central role of Facebook in political discussion in Cambodia, where the government has shut the main opposition party, arrested its leader and cracked down on media and civil rights groups over the past year.
Sam Rainsy's petition against Facebook said the platform had been used by the government to make death threats and alleged that state money had been spent in advertising on Facebook, where Hun Sen's backers have built a strong presence.
Facebook's public relations department has not responded to Reuters requests for comment on the case.
Sam Rainsy lives in France, where he fled in 2015 after a conviction for defamation that he says was politically motivated. He did not respond immediately to Hun Sen's comments on Tuesday.
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