Yeah, Onion has it right. If you are active in twitter and are tweeting during the next UFC event, you might put #UFC157 at the end of your tweets. Everyone else tweeting about it will do the same thing and you can find what others are saying.
The other thing it's used for is to put thoughts into context without having to come right out and say it. Something like: "Internet trends are the worst #ihatehashtags"