Last week, Scout Willis, the daughter of actors Bruce Willis and Demi Moore, went topless on Twitter in protest against the photo-sharing service Instagram's restrictive policy on nudity.

The 22-year-old posted a series of photographs of herself walking through New York with her nipples exposed. Willis captioned one topless photo: "Legal in NYC but not on @instagram" and used the hashtag "#freethenipple" beneath another.

Twitter allows such content because it has a significantly less stringent attitude to naked images. So which social media networks allow nudity and which do not?

Instagram

In its terms of use, Instagram states that users "may not post violent, nude, partially nude, discriminatory, unlawful, infringing, hateful, pornographic or sexually suggestive photos or other content".

Yet sexually suggestive photos abound on the service. The Independent's Lynne Enright notes that Willis "takes umbrage with the fact that Instagram seems fine with the objectification of women, allowing pictures of semi-nude women with captions like 'big titty broads', as long as a nipple tassel (real or digital) covers the areola".

Facebook Facebook's terms state that "Facebook has a strict policy against the sharing of pornographic content and any explicitly sexual content where a minor is involved. We also impose limitations on the display of nudity". But it goes on to clarify that in spite of this, "we aspire to respect people's right to share content of personal importance, whether those are photos of a sculpture like Michelangelo's David or family photos of a child breastfeeding".

Facebook has long struggled to determine what kinds of images it should allow. In 2012, the company drew criticism when a leaked document revealed the site's moderators had been told to ban images of breastfeeding if the nipples were exposed. Simultaneously, the site allowed "graphic images" of animals if shown "in the context of food processing or hunting as it occurs in nature" as well as "deep flesh wounds" and "crushed heads/limbs", but no bodily fluids if the person is visible.

Twitter Twitter is the most permissive of the large social networking sites. It instructs users: "You may not use obscene or pornographic images in either your profile photo, header photo, or user background", but allows users to post pornography in their feed.

Google+ Google's social network does not allow sexually explicit material, or anything that drives traffic to pornographic sites. Its terms of use document says: "Do not distribute content that contains nudity, graphic sex acts, or sexually explicit material. Do not drive traffic to commercial pornography sites. Your Profile Picture cannot include mature or offensive content. For example, do not use a photo that is a close-up of a person's buttocks or cleavage".

Why does Instagram care?

In May, the Financial Times reported that the fashion house Michael Kors had become the first brand ever to advertise on Instagram. The social network is a perfect place for brands to spend their money, Enright says because "its loyal users check it daily and are, on the whole, an image-obsessed, consumer goods-buying bunch".

Despite the feeling of intimacy the network creates, Instagram has an economic imperative to maintain its image. "It's about to start making massive amounts of money, and there's no way the company would ever #freethenipple if it meant jeopardising that," Enright concludes.