One former senior foreign policy official in the Obama administration received messages from someone on LinkedIn offering to fly him to China and connect him with “well paid” opportunities. A former Danish Foreign Ministry official got LinkedIn messages from someone appearing to be a woman at a Chinese headhunting firm wanting to meet in Beijing. Three middle-aged men showed up instead and said they could help the former official gain “great access to the Chinese system” for research. A former Obama White House official and career diplomat was befriended on LinkedIn by a person who claimed to be a research fellow at the California Institute of Technology, with a profile page showing connections to White House aides and ambassadors. No such fellow exists. Foreign agents are exploiting social media to try to recruit assets, with LinkedIn as a prime hunting ground, Western counterintelligence officials say. Intelligence agencies in the United States, Britain, Germany and France have issued warnings about foreign agents approaching thousands of users on the site. Chinese spies are the most active, officials say. “We’ve seen China’s intelligence services doing this on a mass scale,” said William R. Evanina, the director of the National Counterintelligence and Security Center, a government agency that tracks foreign spying and alerts companies to possible infiltration. “Instead of dispatching spies to the U.S. to recruit a single target, it’s more efficient to sit behind a computer in China and send out friend requests to thousands of targets using fake profiles.” The use of social media by Chinese government operatives for what American officials and executives call nefarious purposes has drawn heightened scrutiny in recent weeks. Facebook, Twitter and YouTube said they deleted accounts that had spread disinformation about the Hong Kong pro-democracy protests. Twitter alone said it removed nearly 1,000 accounts. It was the first time Facebook and Twitter had taken down accounts linked to disinformation from China. Many governments have employed similar playbooks to sow disinformation since Russia used the tactic to great effect in 2015 and 2016. LinkedIn, owned by Microsoft, is both another vehicle for potential disinformation and, more important, an ideal one for espionage recruitment, American officials say. That is because many of its 645 million users are seeking employment opportunities, often from strangers. To enhance their prospects, many former government employees advertise that they have security clearances. LinkedIn is also the only major American social media platform not blocked in China because the company has agreed to censor posts containing delicate material. Chinese agents often make offers over various channels, including LinkedIn, to bring the prospective recruit to China, sometimes through the guise of a corporate recruiting firm offering to pay them for speaking or consulting engagements or aid in research. From there, agents develop the relationship. “The Chinese want to build these options with political, academic and business elites,” said Jonas Parello-Plesner, the former Danish Foreign Ministry official who reported the apparent recruiting attempt by the Chinese that began over LinkedIn. “A lot of this thrives in the gray zone or the spectrum between influence-seeking and interference or classical espionage.” People who have just left government are especially vulnerable because they are often looking for new employment, he and other former officials say. Nicole Leverich, a spokeswoman for LinkedIn, said the company proactively finds fake accounts to remove and has a team that acts on information from a variety of sources, including government agencies. “We enforce our policies, which are very clear: The creation of a fake account or fraudulent activity with an intent to mislead or lie to our members is a violation of our terms of service,” she said. Some photographs on fake accounts are generated by artificial intelligence, The Associated Press reported. In multiple recent cases, LinkedIn proved to be an effective recruiting tool. A former employee of the C.I.A. and Defense Intelligence Agency, Kevin Patrick Mallory, was sentenced in May to 20 years in prison for spying for China. The relationship began after he replied in February 2017 to a LinkedIn message from a Chinese intelligence agent posing as a think tank representative,the F.B.I. said. The Justice Department last October charged a Chinese intelligence agent, Yanjun Xu, with economic espionage after he recruited a GE Aviation engineer in a relationship that began on LinkedIn, according to the indictment. Mr. Evanina, the counterintelligence chief, told Reuters last yearthat Chinese agents were contacting thousands of people at a time on LinkedIn. “It’s the ultimate playground for collection,” he said. That level of activity has not dropped, though Mr. Evanina declined to give statistics. “People in the private sector and academia are also being targeted this way,” he said this month. “Foreign intelligence services are looking for anyone with access to the information they want, whether classified or unclassified, including corporate trade secrets, intellectual property and other research.” The Chinese Foreign Ministry did not respond to a request for comment. The former Obama senior foreign policy official, speaking on the condition of anonymity for fear of jeopardizing future interactions related to China, described in interviews a monthslong recruitment effort by someone who appeared to be a Chinese spy. In May 2017, five months after the official left his government job and just after he made a trip to China, someone called Robinson Zhang reached out via LinkedIn.