Goodness, I don't even know. I suppose hashtags actually have a purpose on other social networks, but definitely not on facebook. However, you'll still see whores putting hashtags on fucking everything.
A hashtag is simply a word following a # symbol. It can be whatever you please, but definitely doesn't have to relate to your post AT ALL. So, if you've come to find what a hashtag is, don't even bother. Just hashtag your shit like this, but on facebook it's not going to do a thing.
duckface photo with beverage in hand
So here I am at #starbucks #withtheboo getting my favorite #caramellatte on my #freeperiod. #lovethisbitchtopeices #yolo #juntinbeiber #1dforever. #facebook hashtags for life!