Many of you have noticed #Hashtags on Facebook has come. It is very useful when your post is public. When people search for that hashtags then they may discover your post. This will help in popularity and reach of your post. This is what you wanted when you make your post public! The hashtags on Facebook do the same what it does on Twitter.
Currently, Facebook has not provided Hashtag search using language. So, if I search for say, PHP hashtag, then results in all language related to PHP which is marked using #PHP hashtags will come as result. This cannot work as expected for many. We do not know all languages of the world.