A Instagram post showing a rape and death threat has been accidentally used in an advert on Facebook. Carl Court/Getty Images

A rape and death threat sent to a user has been accidentally used to advertise Instagram on Facebook. The offending advert showed one of the user's Instagram posts containing a screenshot of a threatening email, which read: "I will rape you before I kill you, you filthy whore!"

Olivia Solon, a reporter for UK newspaper the Guardian, was alerted to the ad by her sister for whom it was displayed as a promotional tool on Facebook. "See Olivia Solon's photo and posts from friends on Instagram," read a caption below the shocking image.

"We are sorry this happened – it's not the experience we want someone to have," Instagram said in a statement.

"This notification post was surfaced as part of an effort to encourage engagement on Instagram. Posts are generally received by a small percentage of a person's Facebook friends."

Solon originally posted the image to the Facebook-owned social photography platform in 2016 to highlight online hate speech directed towards women.

"This is an email I received this afternoon," she wrote (via The Guardian). "Sadly this is all too common for women on the internet. I am sure this is just an idiot rather than any kind of credible threat but it's still pretty vile."

The post received three likes and multiple comments, which it appears was enough 'social engagement' to entice Instagram's algorithms into using the photo as a promotional aid for Solon's friend list on Facebook.

The social snafu is the latest in a long line of algorithm hiccups for Facebook, and coincides with broader concerns over hate speech and advert campaigns on the platform.

Earlier today, Facebook CEO Mark Zuckerberg announced that the social giant has agreed to turn over roughly 3,000 ads to congressional investigators that were allegedly purchased by Russian-linked groups and designed to influence the outcome of the 2016 US presidential election.