Mohammad Jan Rasoulyar wrote the message to Ashraf Ghani on the social network to say 90 soldiers had been killed in the past two days in Helmand.
He warned the president that the province could fall to the Taliban.
Mr Ghani's entourage was not telling him the reality of the situation, he added.
Facebook "isn't a good source to deliver the message", he admitted, but "Helmand will collapse to the enemies and it's not like Kunduz, where we could launch an operation from the airport to retake it. That is just impossible and a dream," he wrote.
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Mr Rasoulyar appealed directly to the president for intervention in the province.
"Everyone loves power, I also like my job, but to protect my job I am not ready to pay a huge price," he wrote.
"Be quick and act on this! Protect Helmand from this life and death situation and distance yourself from the circle of those lawyers who tell you everything is OK and the situation is normal."
Sediq Seddiqi, a spokesman for the Afghan interior ministry, told the BBC that the ministry would release a statement on the death toll quoted by Mr Rasoulyar.
A spokesman for one of Helmand's security departments, who wished to remain anonymous, confirmed the number of casualties.
A deputy governor in Afghanistan has made a public plea to the president on Facebook for help in fighting the Taliban.