A man with eight other wives won the bid and paid the girl's father 500 cows, two luxury cars, two bikes, a boat, mobile phones and $10,000 in cash.

"A 16-year-old South Sudanese girl was sold off for marriage to the highest bidder on Facebook in November and a businessman from #SouthSudan outbid four others - which included a senior Sudanese government official," the AfricanFeminism said in a tweet late Wednesday.

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The viral Facebook post was "the biggest test of child abuse, trafficking and auctioning of a human being", said Philips Anyang Ngong, a human rights lawyer who tried to stop the girl's sale.

He called for all of those involved, including Facebook, to be held responsible.

Human rights organisation Plan International South Sudan has also condemned the use of the social media site for the girl's auction, and compared it to modern-day slavery, the report said.

"This barbaric use of technology is reminiscent of latter-day slave markets," George Otim, the organisation's director for South Sudan, was quoted as saying.

"That a girl could be sold for marriage on the world's biggest social networking site in this day and age is beyond belief."

The Human Rights Watch in a 2015 report, noted that 40 per cent of girls in sub-Saharan Africa marry before they turn 18. It added that 15 of the 20 countries with the highest rates of child marriage are in Africa.

The rates are especially high in some countries, with 77 per cent of girls in Niger being married before they are 18 and 60 percent in Central African Republican and Chad.

"Without significant actions to curb the practice, the number of girls married as children is expected to double by 2050," the report noted.

Earlier this month, Facebook was accused of encouraging grooming by offering teenage girls middle-aged men as 'friend suggestions'.

Teenage girls, as young as 13-year-olds, who join the social network are given up to 300 suggestions for who they can add as friends, some of which include middle-aged men who are topless in their profile photos.

Facebook has said that was not a typical experience for teenagers for signing up for the service and that it has safeguards built into its recommendation system.

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