There's been a lot of study on effective hashtag use in terms of the number that can be effectively used. Here's the latest.
Twitter - 2 hashtags work best, with the amount of engagement decreasing with the more you add. That said, there's 100 % more engagement if you use them, and tweets with them are more likely to be retweeted as well. Remember not to trivialize your hashtag, since they're best used to categorize a tweet.
Facebook - It's been found that hashtags actually decrease engagement. Don't use them.
Instagram - Contrary to all other networks, the more you use, the better since on Instagram, hashtags are used primarily to build community. 11 hashtags or more get the most engagement.
Pinterest - Once again, it's been found that hashtags actually decrease engagement. Stay away from them here.One thing to remember is that they now turn up in a Google search and many people do search for them, so they're worth using.
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