Sometimes the tools being used to access the job board feed and post jobs on Facebook or Twitter do not update in a timely manner. This may leave jobs on your social media sites that are linked to posts that have been removed from the job board (so they look blank.) This is not a problem with the job board. The feed from the job board is dynamically created and is always current.
The problem will resolve itself when the tool you are using updates the posts from the feed. In some cases, depending on the tool, you can manually force this update. Please check the documentation for the feed tool you are using for more information.