Social network Tsu.co claimed this week that Facebook is blocking links to it on all of the social media giant’s platforms, including Facebook Messenger and Instagram. The site allegedly even went back and deleted over one million previous posts that mentioned Tsu.

So what is Tsu, exactly? It’s a very small social media site that claims to share its advertising revenue with users. While sites like Facebook keep 100 percent of ad revenue, Tsu only keeps 10 percent. It gives you 45 percent and splits the rest with the chain of users that invited you to the site. That means there’s a strong financial incentive to invite your friends to join, and that’s where the Facebook blocking comes in.

Because of that revenue sharing incentive, many people’s Facebook feeds were soon full of Tsu invites. Many users reported it as spam, and on September 25, Facebook officially blocked Tsu.

“We’re persona non grata,” Tsu founder Sebastian Sobczak said. “You can type in all sorts of seedy websites, and you can get to them. But not us. We don’t exist.”

Facebook has said that it will unblock Tsu if its users can no longer post on both Tsu and Facebook at the same time. That would prevent the incentivized sharing problem. However, some users still see it as potential censorship.

“Very few people even know about Tsu,” said Caroline Franco, a 28-year-old Colombian model who uses the site. “I don’t believe that Facebook and Instagram want Tsu to go viral. it would cost them a lot of money.”