For Facebook (FB) posts we use their native scheduler, meaning we send the post to them and ask them to publish it on a specific date/time. Then FB is in charge of making these posts public. Unfortunately, once in a while they fail to publish it and the post stays on the list of scheduled posts without publishing and without sending back an error message.
We have implemented safeguards to prevent this from happening which makes the error happen less often but once in a while it may happen! :(
It's a bug we've brought to Facebook and can even happen when you schedule posts directly on their page.
If this happens again please inform us immediately.