Hey folks - thanks for the post. I'm an engineering manager at Facebook and worked a bit on this issue earlier today. As some people have pointed out, we've since pushed an update to the Graph API.
A bug in a recent update to our code caused this unintended availability of some metadata about the audience of posts. Keep in mind, the intended audience of the actual posts wasn't affected. So far, this is the only report we've received about the availability of the metadata, and we addressed the situation within a few hours.
As always, developers must abide by our Platform Policies, including obligations that protect the information they access through our APIs. For example, developers may not use someone's information outside of the application without the person's permission. Additionally, people can control whether applications have access to their information and posts through their settings.