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In an unexpected move, Facebook co-founder and CEO Mark Zuckerberg sided with President Trump in his feud with Twitter co-founder and CEO Jack Dorsey.

After Twitter fact-checked one of Trump's tweets about mail-in-ballots, Trump threatened to end Twitter's immunity from liability (lawsuits) by signing an executive order today.

Trump tweeted that mail-in ballots would be "substantially fraudulent." But Twitter's CEO Jack Dorsey fact-checked Trump's tweet, essentially moderating the president.

"Per our Civic Integrity policy, the tweets yesterday may mislead people into thinking they don't need to register to get a ballot (only registered voters receive ballots). We're updating the link on @realDonaldTrump's tweet to make this more clear," Dorsey tweeted.

Trump's executive order removing Twitter's status as a public forum will not only affect Twitter adversely, it will also affect Facebook and other social media platforms.

Twitter and Facebook are privately owned companies and can not be regulated by Trump. But Trump can remove their protections by changing their status from public forums to publishers.

Publishers such as bloggers, who edit and delete content, are vulnerable to lawsuits and do not receive special protections from liabilities as forums do.

That's why Zuckerberg slammed Twitter's CEO Jack Dorsey, saying Twitter should stop policing what Trump tweets.

"I don't think that Facebook or internet platforms in general should be arbiters of truth," Zuckerberg told "Squawk Box" co-host Andrew Ross Sorkin in an interview that aired Thursday morning. "Political speech is one of the most sensitive parts in a democracy, and people should be able to see what politicians say."

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Dorsey responded to Zuckerberg on Friday, tweeting:

"This does not make us an 'arbiter of truth.' Our intention is to connect the dots of conflicting statements and show the information in dispute so people can judge for themselves."

Ari Fleischer, former White House Press Secretary for George W. Bush, said Dorsey was "incredibly stupid" to correct Trump's tweets. "Where does it end? He hasn't done that to the Iranian president. He hasn't done that to Chinese leaders. Why is he doing it?"