Lawyers for exiled Cambodian opposition figure Sam Rainsy have filed legal action against Facebook, in a case that highlights how the quest for "likes" has gripped the small Southeast Asian country's leading politicians.  The petition in US federal courts asks the social media giant to disclose information related to the popularity of the Facebook page of Sam Rainsy's rival, Prime Minister Hun Sen, whose Facebook page ranked third in global engagement among world leaders, according to public relations firm Burson-Marsteller.  The petition accuses Hun Sen of using state money to buy Facebook ads and says he levelled death threats against Rainsy in violation of the platform's policies, including a speech posted to his page last week vowing to attack opposition members with rocket launchers.  "Facebook should act to prevent its platform being manipulated to help prop up dictators," said Rainsy's lawyer, Richard Rogers...Rainsy...accuses Hun Sen of purchasing support in countries such as India and the Philippines, where the prime minister's page is popular and which are home to "click farms" that produce fake followers for social media accounts...Cambodian courts found Rainsy guilty of defamation in November 2016 after he accused officials of creating fake accounts to support Hun Sen's page.  The filing in Northern District of California federal court asks Facebook to disclose information about the authenticity of Hun Sen's likes, as well as communications he may have had with associates on the platform on a variety of topics...In recent years the Cambodian government has stepped up its prosecution of Facebook critics...