Your Social Headers or “Covers” Are a Big Opportunity
On Facebook, anyone coming to your page sees your header (also called a cover) first and can't avoid it. That means it's a great opportunity to communicate what you're about.
Your header doesn't have to blast a sales message, although some people do include their latest offer in them. I prefer to create headers that fit your brand so they help people get familiar with your flavor. That doesn't mean don't think them through thoroughly, which is why this offer is for a set of them, so you can rotate them.
LinkedIn also uses a header and you can do nice things with it. Headers on mobile appear much smaller, obviously, so I design keeping that in mind, as well as the area and dimensions required for each device.