She appealed the decision, and after two weeks finally received a response from Instagram's parent company Facebook, which said the ad was rejected on the grounds that it violated its advertising policies.
"We don't allow ads that focus on aspects of a person's body to highlight an undesirable or idealised body state," a screenshot of the response posted on Ms Gillies' blog read.
The policy went on to list several examples of "undesirable" body depictions that included baldness, cellulite and acne, prompting Ms Gillies to start #UndesirablesOfInstagram and encourage her followers to post pictures of their own "undesirable" faces.
Facebook's policy is designed to avoid users feeling "singled out" and target scam ads hawking miracle cures.
"This ad was rejected in error and we are sorry for the mistake," a Facebook spokesman told news.com.au.
"Over the past 18 months we have grown our safety and security team from 10,000 to 30,000 people.
"Fifteen thousand of those are dedicated to reviewing content in line with our community standards."
That equates to about one community standards content reviewer per 140,000 people who use at least one Facebook product (Facebook, Instagram, WhatsApp, Messenger) every day.
This ratio goes some way towards explaining why it took more than three weeks for Ms Gillies to have her promoted post approved by a human reviewer, by which point the exhibition it was meant to promote had ended.
Ms Gillies told The Verge she participated in the exhibition to get people talking about rosacea, and her Instagram was largely focused on the normalisation and acceptance of skin conditions and promoting skin positivity.
Last year, Instagram partnered with the Butterfly Foundation, an Australian organisation that supports people with eating disorders and negative body image.
But while Instagram is trying to publicly support and promote body positivity, it remains a challenge for the social media platform to effectively moderate content posted by its massive user base to ensure its policies don't cause harm to the people they're intended to protect.
PROOF SOCIAL MEDIA IS BAD FOR YOU
Body image and self esteem have emerged as growing concerns on social media. If you've ever felt sadder and more isolated logging off Facebook than you did when you logged on, take comfort in knowing you're not alone.
A 2017 study of more than 5000 adults found using Facebook had an overall negative impact on wellbeing and the detrimental impacts of social media use actually negated or outweighed the positive benefits of face-to-face social interaction.
The research was conducted over three years and published in the American Journal of Epidemiology.
Past research found people who had regular, close and real-life social interactions were healthier, happier, drank less and lived longer.
Meanwhile research into online interaction showed it made us fatter, lazier and eroded our self esteem.
One of the reasons is the constant comparisons made between ourselves and the people we follow on social media, of whom we only see a highlight reel of their most impressive moments.
Study participants reported the more they used Facebook, the worse they felt, both physically and mentally, but the study did find those who interacted more with close personal friends fared better.
They also reported a higher level of life satisfaction.
The study relied on self-reported data, meaning the problem could be even more widespread than the research indicates.
