Around 40 people were said to have been watching the stream at one point but nobody reported the incident to police.
A police spokesman said authorities first learned of what happened after the girl's mother approached the head of police as he was leaving a station.
Detectives have questioned several people but no arrests have been made.
The video has since been removed.
The girl, who had been missing for a day, has now been found by detectives and reunited with her family.
Her mother, whom the BBC is not naming, said her daughter appeared to be scared in the footage, adding "it's so disgusting".
"It's just terrible. I know they'll get their day coming," she said on Tuesday morning, before her daughter was found.
Chicago police chief, Supt Eddie Johnson, was said to be "visibly upset" after the girl's mother showed him screenshots of the video.
A relative of the girl, Reginald King, says he was the last to see her before the alleged attack, after the two attended church together on Sunday.
"Nobody deserves that. No human being deserves for that to happen to them," he told local media, adding that he suspects local "thugs" committed the attack.
After the girl was found, she was taken to hospital with injuries to her face, he told the Chicago Tribune.
A spokesman for Chicago police tweeted on Tuesday afternoon that police are "making good progress" in identifying the assailants.
"Interviews ongoing but no formal suspects named yet,", Anthony Guglielmi added.
"Crimes like this are hideous and we do not allow that kind of content on Facebook," a spokeswoman for the company said.
"We take our responsibility to keep people safe on Facebook very seriously and will remove videos that depict sexual assault and are shared to glorify violence."
In January, Chicago police arrested four people following a separate incident in which a man's alleged assault was live streamed, also on Facebook Live.

India-born Rajat Gupta had leaked boardroom secrets to Raj Rajaratnam, a former hedge fund manager who is serving 11 years in prison.
Gupta, 67, was released on Friday, the Press Trust of India said, quoting his record at Federal Bureau of Prisons.
The US court has agreed to re-hear his appeal against his 2012 conviction, reports say.
Gupta completed the last two months of his jail term at his home in Manhattan, after being released in January from a correctional facility.
He was confined to his apartment and was required to wear an ankle bracelet that monitored his movements, the Press Trust of India reports.
During the court case, which resulted in Gupta being found guilty in June 2012, the jury heard secret recordings of conversations between him and Rajaratnam.
The trial focused on a phone call made to Rajaratnam on 23 September 2008, minutes after Gupta had listened to a private conference call discussing a $5bn (Â£3.4bn) investment in Goldman Sachs by Warren Buffett's company Berkshire Hathaway.
The deal was due to be made public after stock markets closed that day.
According to phone records, Rajaratnam bought $40m in Goldman Sachs stock moments after the phone call, earning nearly $1m.
Apart from the two-year prison term, Gupta was also fined $5m.
Gupta, who was born in India and educated at Harvard, also served on the boards of Procter & Gamble, the Rockefeller Foundation and the Bill and Melinda Gates Foundation.
The alleged sexual assault of a 15-year-old girl by five or six males was streamed on Facebook Live, according to Chicago Police.
[NEXT_CONCEPT]
A former Goldman Sachs board member has been released after serving a two year sentence for insider trading in the US.