Facebook Inc. has been allowing advertisers to target users in mainland China, despite a ban on the company’s social networks there that prohibits citizens from accessing the apps.

Facebook has long said it works with Chinese advertisers to reach users only outside the country, not inside. But the company’s advertising system tells a different story. Through the ad-buying portal, businesses are told they can reach 3.7 million people in mainland China on Instagram, the photo app Facebook owns. Advertisers can target specific cities including Beijing, Shanghai and Tianjin.

Facebook said this is not a mistake. "There are various technical ways a very small fraction of people in China may be able to access Facebook and see ads,” the company said in a statement. For example, it cited those who use international mobile roaming or otherwise have access to an international connection. The social network has been blocked in China since 2009.