People who post hastags might also be posting to twitter which may be why.
Hashtags are just like tagging, but for sites like Twitter and Facebook, and for a few exceptions (like wanting to make sure certain people see something you need or want to be seen) I don't get them.
I hate how trends move and die so quick nowadays. They explode with popularity, the media gets all over it, other people get tired of it, and people who hardly got a chance to enjoy it get labeled as annoying or obnoxious for liking it. Like Gangnam Style. I actually liked that song, and not to sound like a hipster, but I was liking it before it trended. And now I'm obnoxious for liking it.