Abigail Kopf was shot February 20 in the parking lot of a Cracker Barrel restaurant.
"Our girl is fighting ... and winning!" an update on the page exclaimed.
Abigail was one of eight people allegedly shot by Jason Brian Dalton, who police have said opened fire at three sites.
Abigail's friends -- Dorothy Brown, 74; Barbara Hawthorne, 68; Mary Lou Nye, 62; and Mary Jo Nye, 60 -- were killed. Abigail was in the passenger seat of one of the women's vehicles.
Tiana Carruthers, the first person shot during the gunman's spree, is in serious condition but improving, Kalamazoo County Undersheriff Paul Matyas told CNN. The undersheriff said authorities expect the hospital will upgrade her condition soon.
She is still at Borgess Medical Center.
Dr. Aaron Lane-Davies, of Bronson Methodist Hospital, told CNN affiliate WOOD that on that first day Abigail's heart stopped and doctors got it beating again.
Abigail was initially kept on life support so her organs could be donated. But one day, while her mother, Vicki, was holding her hand, she suddenly squeezed back.
The mother described the moment as "breathtaking. ... A miracle on its own."
Abigail's parents have said their daughter is strong-willed.
Dalton, 45, is charged with six counts of murder in the nearly five-hour rampage.
Police have said Dalton does not have a history of mental health issues.
Kalamazoo Public Safety Chief Jeff Hadley told CNN last week that Dalton legally bought the 9 mm handgun he allegedly used in the shootings, though he did not have a concealed pistol license.