First, it's always a good idea to check the Joomla Extension Directory if you're looking for a plugin/extension. You'll never know what you can find if you don't look. ;)
Second, you may not need a plug in for this. Assuming that you have:
- a code snippet with instructions on how to embed it
- access to your site's template files if needed
you can just use a module or two to get things working. This isn't just for Facebook ad embeds, but any kind of embed that has a script and some target markup.
Now, if you really want to use a plugin, and you can't find one that works, it is possible to write you're own. This will require knowledge of PHP, Object Oriented Programming, Joomla's extension types, and Joomla's plugin events and types. You can get a crash course in plugin development by googling "Joomla Plugin Development." If you want, you can also hire someone to develop a plugin for you. Be warned, though, that can be very expensive.
I hope this helps out, I've never heard of embedding Facebook ads before, and I couldn't really find any info on it, so I have made a few assumptions on how it is supposed to work. If I have made some incorrect assumptions, let me know and I'll do my best to edit this answer to better fit what you actually need.