Facebook has updated its policies for third-party application developers, which now cover the case where Twitter's new Vine video-sharing app is unable to access Facebook's friend-finder tool. Facebook's Justin Osofsky, director of platform partnerships and operations, said the "clarifications" were published after the site received questions about its policies over the past few days.

"For a much smaller number of apps that are using Facebook to either replicate our functionality or bootstrap their growth in a way that creates little value for people on Facebook, such as not providing users an easy way to share back to Facebook, we?ve had policies against this that we are further clarifying today," Osofsky wrote in a blog post.

The changes explain why Facebook decided to block the app, a move that sparked a wave of criticism this week.

Twitter launched Vine on Thursday, which includes a tool that allows users to search for their Facebook friends and add them to their Vine network. Vine also lets users automatically share the videos they create with the app with their Facebook friends. However, Facebook friend-search tool had been disabled. Vine users get an error message when they try to perform the search.

One of the amended sections of FAcebook's platform policies reads: "You may not use Facebook Platform to promote, or to export user data to, a product or service that replicates a core Facebook product or service without our permission." The policies don't say what, exactly, constitutes a "core" Facebook product.

The new policies say developers can build their own social network via Facebook's API, but only if the app allows users to share their experiences back with Facebook users.

They also say that if Facebook disables an app, the developer of the app must delete all the user information it collected through Facebook's API, unless it is basic account information or it receives consent from the user to retain it.