Facebook has changed a setting again - but this time, instead of compromising your security, it's really protecting it.
Every now and then, you'll have to use a Facebook account to sign in to an app. Maybe without knowing it, you've implicitly allowed that app to post to Facebook for you - and that's a sort of free ad for them.
Next time you check your Facebook feed, you see this mysterious post that you didn't ask for. It's frustrating.
Now, Facebook won't show as many of those implicit third-party posts on your News Feed. It won't stop them outright, but it's a start.
If you explicitly tell a third-party app to post for you, it will show up in your News Feed like normal.