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Among its many virtues and drawbacks, social media may be one of the most effective tools ever invented to assist journalists in harming their careers.

Since the advent of Twitter, Facebook and other instantaneous digital platforms, reporters have lost their jobs, been suspended or been reassigned after posting things deemed inappropriate by readers, viewers and — most important — their bosses. The objectionable posts have usually called into question the journalists’ ability to remain neutral and fair to both sides of any story.

The latest casualty: CNN correspondent Diana Magnay, who last week stirred criticism for a tweet about a group of Israelis who were cheering a missile attack on Gaza. Magnay said in her tweet that members of the group had threatened her. “Scum,” she concluded. Amid an outraged reaction, the network apologized, saying Magnay was referring only to the group’s alleged harassment of her, not to its support of the military action. She was quickly reassigned to Moscow.

The incident echoed CNN’s dismissal in 2010 of Octavia Nasr, a longtime foreign-affairs editor. The network cut Nasr loose after she tweeted her thoughts about the death of a leader of Hezbollah, the Lebanese terrorist organization, calling him “one of Hezbollah’s giants I respect a lot.”

News organizations have encouraged their journalists for years to engage with readers on social media to promote their stories, gather information and foster “community.” The problem is that social media also makes it easy for reporters to unburden themselves of their innermost thoughts and opinions, without editors standing in the way and often without much context.

Although journalists sometimes add disclaimers such as “All opinions my own,” their followers and employers sometimes object, saying such opinion-slinging displays bias. Most organizations deal with such episodes internally, but many spill into public view. Among others:

●The Ashland (Ky.) Daily Independent this month fired a veteran reporter, Ken Hart, after he used his Facebook page to critique a local car dealer’s TV commercial. Hart said the newspaper acted after the dealer complained about the post and apparently threatened to pull his ads from the newspaper.

●MSNBC fired an unidentified employee in January who used the cable network’s Twitter account to get in a political dig while tweeting about a Super Bowl ad for a breakfast cereal. The tweet — “Maybe the rightwing will hate it, but everyone else will go awww: the adorable new #Cheerios ad w/biracial family” — brought a rebuke from the Republican National Committee and an apology from MSNBC.

●St. Louis TV station KMOV (Channel 4) fired one of its top anchors, Larry Conners, after he implied in a series of Facebook posts last year that he and his wife had been targeted by the IRS because of tough questions he’d asked President Obama in an interview. Conners implied that the IRS’s actions were payback for his asking Obama during a 2012 interview about presidential travel expenditures. The station said Conners was dismissed for “taking a personal political position” on one of the station’s Facebook pages that “made it impossible for him to report for KMOV on certain political matters going forward without at least an appearance of bias.”

The Washington Post has had its issues, too. In 2010, sports columnist Mike Wise was suspended for a month for a stunt-gone-wrong tweet about Pittsburgh Steelers quarterback Ben Roethlisberger. Other news staffers have been warned about expressing opinions.

Nasr, who was fired by CNN, said in an interview that she doesn’t regret her controversial tweet but does regret not saying what she meant in more detail. She said the subject of her tweet — Mohammed Hussein Fadlallah, a Lebanese cleric — had broken with Hezbollah on many issues, including women’s rights, and had condemned the Sept. 11, 2001, terrorist attacks on the United States. In hindsight, said Nasr, now a columnist, she would have sent out a series of tweets explaining her position. “I can’t provide context in 140 characters,” she said. “That was my mistake at the time.”

News organizations typically spell out in detail what’s acceptable on social media for their employees. The Associated Press’s internal guidelines, for example, are seven pages long and cover the rules for retweeting, deleting tweets and “trash-talking,” among other things.

Reuters notes in its “basic principles” that “Journalists are people too, with all the rights of citizens. If we want to tweet or post about a school play, a film or a favorite recipe, we are free to do so. When dealing with matters of public importance and actual or potential subjects of coverage, however, Reuters journalists should be mindful of the impact their publicly expressed opinions can have on their work and on Reuters.”

NPR’s lengthy guidelines warn, “Everything you say or do in a social media environment is effectively a public statement from an NPR journalist.” Politico asks its reporters not to break news on social media; the news should first appear on Politico.

In a “reminder” to newsroom employees in late 2012, the New York Times’ standards editor, Philip B. Corbett, wrote, “We should always treat Twitter, Facebook and other social media platforms as public activities. . . . Civility applies whether an exchange takes place in person, by telephone, by letter or online.”

Corbett said he wrote the memo after an extensive and often nasty Twitter exchange between a Times contributor, Andrew Goldman, and Jennifer Weiner, a best-selling novelist who had criticized Goldman’s work. The Times suspended Goldman for his tweets.

“We’ve kept our guidance pretty broad,” Corbett said in an interview. “We have very occasionally had to remind people about this, but given how many journalists we have and how active many are on social media, I would say there have not been many significant problems.”