Are you going to add Facebook Hashtags?
Unfortunately, Facebook doesn’t provide any opportunity to display hashtags via API requests. So its technically impossible to do this right now. We will add this feature for sure as soon as Facebook opens this possibility for developers.
As some workaround, you can create Facebook feeds and add automatic filters https://docs.social-streams.com/article/71-automatic-moderation-with-filters by keywords or hashtags. This will not search through all Facebook, but at least you will be able to filter posts from pre-defined pages (feeds).