Facebook took the post down but not before it was shared about 26,000 times, data showed.
"We remove incorrect information about the severity of Covid-19," a company spokesman told Reuters.
The world's largest social media company, which exempts politicians from its third-party fact-checking programme, has rarely taken action against posts from the president.
Twitter disabled retweets on a similar tweet from Trump yesterday and added a warning label that said it broke its rules on "spreading misleading and potentially harmful information related to Covid-19" but that it might be in the public interest for it to remain accessible.
During the 2019-2020 influenza season, the flu was associated with 22,000 deaths in the United States, according to US Centres for Disease Control and Prevention estimates.
Since the first case of coronavirus was recorded in the United States at the beginning of this year, more than 210,000 people in the country have died, the world's highest death toll.
On Monday, Trump told Americans "to get out there" and not fear Covid-19 as he returned to the White House after a three-night stay in the hospital where he was treated for a coronavirus infection.
Twitter, which has been using labels to flag tweets with misinformation - including from the president - told Reuters it is currently trying to respond more quickly and more overtly.
Facebook removed a Trump post for coronavirus misinformation for the first time in August. The post included a video in which the president falsely claimed that children were "almost immune" to Covid-19.
Biden warns there 'is a lot ot be concerned about'
Democratic presidential nominee Joe Biden has criticised Trump for downplaying the severity of coronavirus, saying there is "a lot to be concerned about".
He said the president should be communicating the "right lesson" on masks and social distancing.
It came after Trump told Americans not to fear Covid-19.
He has returned to the White House to continue his treatment for coronavirus after a three-night hospital stay.
The president, who is still contagious, removed his mask on the balcony of the White House while posing for pictures.
While he is no longer in hospital, his doctor has said he "may not entirely be out of the woods yet".
Speaking at an NBC television town hall event in Miami, Florida, on Monday night, Biden said he was "glad" that the president seemed to be recovering well.
Democratic presidential contender Joe Biden at the forum in Miami. Photo: AFP
But, he said, "I would hope that the president, having gone through what he went through... would communicate the right lesson to the American people. Masks matter."
"The only thing I heard was one of the tweets saying that, you know, don't be so concerned about all this, essentially.
"There's a lot to be concerned about. Two hundred and ten thousand people have died," he added.
The US has recorded more coronavirus cases and deaths than any other country in the world.
Former First Lady Michelle Obama also criticised the president's response to the virus in a video released yesterday in which she urged people to vote for Biden "like your lives depend on it".
"Seven months later, he [Trump] still doesn't have a plan for this virus. Seven months later, he still won't wear a mask consistently and encourage others to do the same - even when those simple actions could save countless lives. Instead, he continues to gaslight the American people by acting like this pandemic is not a real threat," she said.
Trump's diagnosis has upended his campaign for a second term in office, less than a month before the 3 November presidential election.
Two more cases in White House
Another two White House staffers have tested positive for Covid-19 a day after the US president returned to the White House after being hospitalised with the highly contagious disease.
One of Trump's valets, an active member of the US military who travelled with the president last week and a military aide to the president have both reportedly caught the virus.
The White House says it will take more precautions to protect staff around Trump.
Meanwhile, the US military's Joint Chiefs of Staff have gone into self-quarantine after the Coast Guard's No 2 tested positive for the novel coronavirus following a top-level meeting at the Pentagon last week, US officials said.
Mark Milley Photo: AFP
US defence officials, speaking on condition of anonymity, stressed that the military's top brass - with the exception of the Coast Guard vice commandant, Admiral Charles Ray - had all tested negative so far and were still carrying out their duties.
But the disclosure risks adding to a sense of uncertainty about operations at the highest levels of the US government after Trump himself contracted the illness, along with senior White House staff and other Republican leaders.
In a statement, Pentagon spokesman Jonathan Hoffman said that all potential close contacts from Ray's meetings at the Pentagon were tested yesterday and that none of them exhibited symptoms.
Ray attended meetings late last week with the US military's top brass, including the chairman of the Joint Chiefs of Staff, Army General Mark Milley, Trump's top military advisor.
US officials told Reuters that Milley was self-isolating, as was the vice chairman of the Joint Chiefs of Staff and military leaders from the Army, Navy, Air Force and Marines.