The actress, 31, was joking about pre-show jitters, but commenters found very little funny about her body.
The image showed Chris Pratt's ex-wife standing near a bin, wearing a long sleeve top, shorts and black socks.
The concern was due to her exposed thighs which looked very slim.
"Having pre-show jitters- so glad Michael Sherman captured it- also I eventually decided to put on pants," Faris penned.
"You look so unhealthy, eat," one person shared, while another said, "This is alarming!" and "I feel this pic is a cry for help, I hope you get the help you need."
As a response, the mum-of-one deleted the image.