I've never really been clear on why I'm supposed to use hashtags on Twitter. Apparently, they're supposed to help other people find my tweet using the search function or "trending topics." But, as this great article from the Nieman Journalism Lab at Harvard points out, the volume of tweets is still too high for hashtags to matter. Daniel Victor:
According to Twitter, #SuperBowl was used 3 million times over about five hours on Super Bowl Sunday this year. Look at all those people who might be interested in our jokes about Beyonce! And yet getting any single person’s attention is just short of impossible, like a single Niagara droplet screaming for notice as it shoots down the falls.
Though there were peaks and valleys, 3 million tweets over five hours comes out to an average of 167 tweets per second. To say that someone would have to search for “#SuperBowl” in the split-second you sent it would actually be a little generous; assuming they’ll notice your tweet if it’s in the most recent 10 tweets, users would have a window of 1/17 of a second to find you.
Basically, you're stuck between a rock and a hard place. Use a hashtag that's too common and your tweet will be lost amidst the thousands or millions of others using the same one. A hashtag that's too specific won't be noticed or searched for. A hashtag that's "trending" hides individual tweets almost by definition, since it means there are many people using the same tag.
I also have trouble believing that people do very much searching on Twitter. Anecdotally, I don't see or hear about friends using Twitter's search function often at all. Victor points out that hashtags are occasionally useful at conferences or among other small groups. I agree, to a point. If the conference is too large or high-profile (take South by Southwest for example), filtering by hashtag is still an exercise in reading a lot of junk tweets. Again, it's only useful if you can thread a pretty small needle between too not enough posts and too many.
The problem is compounded by the fact that hashtags feel like marketing. It's as if companies convinced us to append the little trademark (™) symbol every time we write the name of a product. Maybe I'll start posting about Coke™ on Twitter™ while using my Mac™. #Winning! Doing so doesn't add any information to the message. In fact, hashtags detract from it – clogging up my post with what is, essentially, advertising. (Look no further than TV commercials, which now often contain a "suggested" hashtag.) Plus, as Victor points out, they're ugly.
So, to sum up. Hashtags aren't likely to get a tweet more exposure, they don't add any information, and they look bad. Why are people using them, again?