You can do whatever you want with ghost. If you use Ghost (Pro) for hosting, you need to abide by their terms of service, but other than that there are no restrictions as to what can be done with it.
I don’t know how FAN banner ads work, but can provide generic guidance. You can inject code into any post using post code injection (which will be useful if there’s a script you need to add) and you can insert raw HTML into your post. Just make sure to add a couple of linebreaks before returning back to markdown
e.g.
<div id="facebook-ad-spot"></div>
# My super duper title