The latest advice about using hashtags effectively on social media platforms is from Hootsuite in The Do's and Don'ts of How to Use Hashtags. While people now know that "#" is more than the pound sign, Hootsuite asserts that "many people still don’t understand how to use hashtags."
Let's start with why we use hashtags. Again, Hootsuite:
The hashtag is likely the most popular means of categorizing content on social media. It makes your own content discoverable and allows you to find relevant content from other people and businesses. The hashtag also allows you to connect with and engage other social media users based on a common theme or interest.
Consider the Storify #commbuild Mobile Tools for Digital Storytelling in the blog post TA: Digital storytelling. Birgit Pauli-Haack could pull the Tweets from the @CommBuild Twitter chat about mobile tools for digital storytelling by searching for the hashtag #commbuild. It's the same way I was able to Storify the #AskDCDPR Twitter chat (see Miss the #AskDCDPR Twitter Town Hall Mar. 19? Here's the event Storified.). And, it's the same way that someone will be able to Storify Mayor Muriel Bowser's first State of the District Address. BTW, the hashtag for SODA is #SODA15.
Okay, back to the Hootsuite piece. Hootsuite offers specific advice in both the Do's and Don'ts categories. The main Do is to be specific. Other Do's include determining whether the hashtag you want to use is already in use. HOW TO: Effectively Use Twitter Hashtags provides links to online tools to find out.
Which leads us to the Don'ts. Mayor Bowser's campaign clearly did not determine if #FreshStart is already used. Or if they did, did not care that a young woman with a hangover, a model who just got a hair cut, and all sorts of other folks worldwide who use the broad term to mean they are off to a, well, fresh start. The problem for the mayor is that a search of the hashtag does not yield Tweets relevant to her. A more appropriate hashtag for the mayor would be #DCFreshStart or #FreshStartDC.
Other Don'ts include making hashtags too long and using them too often. While hashtags can be used to express your personal saying (think @Dizzyluv25's #Everydayisahustle), not every Tweet requires a tag.
Related to this is the number of hashtags. Linchpinseo.com, in Infographic: Twitter Tweet Cheat Sheet To Increase Engagement, reports that the sweet spot for hashtags is one or two. Engagement increases 21% when one or two hashtags are used and drops 17% when more than two are used. The keys here are "engagement" and hashtags. Not all Tweets are about engagement and thus hashtags will not matter. Too many hashtags, however, is a surefire turnoff.
[UPDATE: Graphic updated 3/29/15, 7:20p.]