WASHINGTON – News this week that two former Twitter employees were charged by the Department of Justice with spying for Saudi Arabia inside the company put a fresh spotlight on a problem few businesspeople think about as they tweet, "friend" and message away on the internet: Social media is crawling with spies.

And the biggest target, according to some experts, isn't the flashy Twitter – it's the buttoned-down site LinkedIn, which is owned by Microsoft.

Current and former law enforcement officials contacted by CNBC argue that LinkedIn's unique combination of professional information and implicit promise of financial gain makes it the perfect place for foreign intelligence services to troll for corporate insiders willing to spill intellectual property for money, or for U.S. government employees who have grown disgruntled in their jobs.

LinkedIn, they say, is likely being targeted by foreign agents looking to infiltrate the company physically as well as by spies looking to use phony LinkedIn accounts to connect with sources.

"If you're a foreign intelligence agency, LinkedIn is a gold mine, because you can get friends, followers, family — and people's rank inside companies," said Clint Watts, a former FBI special agent and senior fellow at the Center for Cyber and Homeland Security at George Washington University. "There are more secrets in Silicon Valley than there are in Washington, D.C."

Former FBI counterintelligence operative Eric O'Neill agrees. To spies, he said, "LinkedIn is interesting — you can use it to find out a lot of corporate information without even hacking."

O'Neill, who played a key role in bringing down the FBI mole Robert Hanssen for spying on behalf of the Soviet Union, said Chinese intelligence agents have been among the most aggressive users of LinkedIn. "Data is the currency of our lives, and companies have all the data."

Current government officials have gone public with warnings about Chinese espionage on LinkedIn. In August, William Evanina, director of the National Counterintelligence and Security Center, told The New York Times that China's spies are operating on a mass scale. "Instead of dispatching spies to the U.S. to recruit a single target," he said, "it's more efficient to sit behind a computer in China and send out friend requests to thousands of targets using fake profiles."

A Department of Justice official told CNBC that the Chinese recruitment efforts have been paying dividends for Beijing. "Of the recent U.S. intelligence officers who've flipped and gone to work for the Chinese, some of them were recruited by LinkedIn," he said.

The problem, the official said, is that government officials, who are themselves looking to network and find higher paying jobs with more responsibility, put detailed accounts of their careers on the site — which can give the Chinese and others a road map of exactly whom to approach.

"It's a site where people put up all their former security clearances and where they used to work," the official said. "People ought to be a first line of defense for themselves and not post things on there that they wouldn't tell directly to a foreign intelligence service."

The good news for the U.S. government, the official said, is that LinkedIn is aware of the problem, and working to solve it. "We've talked to them about it, and they're very responsive," he said. "They're very forward leaning on supporting lawful process."