Hashtags are ubiquitous, and that's probably why their usefulness is nearly dead.
I say dead, not because you can't use them anymore, but because people don't want to use them for what they were designed for: to organize conversation.
People do want to use them, but only to call out emotional states, and for emphasis in lieu of other formatting options.
Using a hashtag for what it was originally meant for on social media has been made synonymous with over-promoting.
In-fact there is an entire meme toiling the fact that we try to over-tag every single activity we do:
This is a huge bummer because they are actually exceptionally useful.
Twitter's entire organizational structure initially relied on hashtags to categorize hot discussions.
Now hashtags are only used (from an organizational standpoint) for trends and events.
Meaning if I were to write a post about content marketing that was extremely insightful, and tweet it with #contentmarketing at the end of it, people would probably look away repulsed by the idea that I used a hashtag to try to sort my conversation to those that might be searching for that keyword, and in so doing that I am over-promoting.
They see marketing and they look away.
Now on the other side of things, if I were at a conference and wanted to tweet an insight, I might be able to get away with using a #SXSW, #WWDC, or #io15 – to give people context of where the information or opinion is coming from or is targeted at. This would be fine, this happens all the time.
Luckily Twitter understands this limited use and has taken their technology to a level to understand the context of someone's tweets, the topic matter at hand, and to sort by that when you search. Now when you search a topic you get related hashtags, and sorted discussion directly from the text of tweets – basically what hashtags were supposed to solve.
Hashtags have simply become the punchline to a post.
The hashtag which was once used to sort discussion is now used as a sarcastic after-thought. The organization of discussions is limited to "discussions" that wouldn't really exist at all had they not been sarcastically invented.
Take The Rock for example:
Who the hell would be searching for discussion about "TheyReinvent" or "LegendsDontRetire" – no one, they wouldn't be.
The Rock knows this, it's not like he's trying to acquire the "TheyReinvent" market of Instagram. It's more of a punch-line use-case than anything. It's a replacement for bold text or italics. It's the bit that stands out at the end of a sentence, it just so happens to be a link that leads to other information of a similar sort of discussion.
The hashtag that does stand out as useful here, however, is #BALLERS – which is in fact a new TV show on HBO.
People searching for information around BALLERS might actually come across this Instagram post from Dwayne, and may be able to gain some personal value from this content. This is the topical use it was meant for, but is rarely actually used for.
Now, that's the larger social web. What about in other applications?
This is where the hashtag is having its renaissance.
Apps like Slack are using hashtags to sort discussions between teams in closed networks, where a ton of discussion can be easily sorted and searched by topic using their respective hashtags.
This is beautiful, this is what hashtags were intended for.
Everyone is searching for the answer for how to use hashtags:
But honestly there is a very simple answer:
If you're trying to organize a discussion around a topic that is trending or an event that is happening, hashtag until the cows come home. If you're trying to organize a general discussion topic, just use normal text – otherwise people will think you're a bloodthirsty marketer up to no good.
Apart from this, people will continue to use hashtags for emphasis in lieu of other formatting options.
Also to tag #FML on Instagram selfies and Tumblr posts.