One wrote: “I wasn’t ready for this forever lively legend to be gone so soon.”
Another added: “A true legend. Thank you for your talent, your animal advocacy, your immeasurable charm. You will be missed.”
“I just heard the news and am so shocked and sad,” penned a third. “Sending my condolences and love to the Keaton family and friends.”
An official cause of death is yet to be confirmed by authorities, who shared an update following her passing.
But close friends of the star have opened up about Keaton’s rapid ‘decline’ in the months leading up to her death.
Speaking to People, the longtime friend said: “She declined very suddenly, which was heartbreaking for everyone who loved her.
“It was so unexpected, especially for someone with such strength and spirit.
“In her final months, she was surrounded only by her closest family, who chose to keep things very private.”
They added: “Even longtime friends weren’t fully aware of what was happening.”
Another of her close confidants was songwriter Carole Bayer Sager, who revealed that the 79-year-old was devastated by the Los Angeles wildfires that destroyed so much of the US city earlier this year.
“I saw her two or three weeks ago, and she was very thin,” Sager noted. “She had lost so much weight.”
The star added: “She had to go to Palm Springs because her house had been damaged inside, and they had to clean everything.
“She was down there for a while, and when she came back, I was kind of stunned by how much weight she’d lost.”