It's believed that up to 40 people watched the incident on the social media platform which shows at least five to six males sexually assaulting the 15-year-old, police spokesman Anthony Guglielmi said.
The video came to light after the girl's mother approached Chicago Police Superintendent Eddie Johnson on Monday. She told him her daughter had been missing for 24 hours, Guglielmi said.
Some people are identifiable from the video, Guglielmi said. The teen knew at least one of her alleged attackers and may have been acquainted with the others.
Police contacted Facebook, Guglielmi said, and the video is no longer available on the site.
A Facebook spokesman said, "Crimes like this are hideous and we do not allow that kind of content on Facebook. 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."
The teenager in question is currently being treated in hospital.
In January, four people were charged in an attack
on a special needs teen that was streamed on Facebook Live.