Because of the 140-character limit on Twitter, space is at a premium. Therefore, tweets with just one hashtag were found to get the most engagement, averaging 90 interactions per post, the report said.
Tweets with multiple hashtags see a sharp decline in engagement. For example, posts with nine, 10 or 11 hashtags typically average engagement of fewer than 20 interactions per post.
Meanwhile, hashtags with 18 characters were found to perform best, with an average engagement of 12,557 interactions per post, followed by hashtags with three characters, with an average engagement of 10,871 interactions per post.
Like multiple hashtags, longer hastags - typically 20 characters or more - see a sharp decline in engagement, the report says.
For Facebook, the impact of adding multiple hashtags to a post "is just as extreme as on Twitter", the report says, despite brands having far more space for their messages.
According to the report, Facebook posts with one hashtag perform best, with an average engagement of 1,771 interactions per post. However, posts with more than one hashtag see a steady decline in average interactions per post, and engagement drops to fewer than 500 interactions for posts with more than six hashtags.
In terms of length, hashtags with six characters perform best, with an average engagement of 4,968 interactions per post. Hashtags that are 10 to 17 characters in length also perform well, but engagement starts to decrease for longer hashtags, the report finds.
Instagram, meanwhile, paints a far different picture to Twitter and Facebook, as posts with nine hashtags were found to perform best.
Instagram posts with nine hashtags had 28,548 interactions per post on average, the report shows.
While engagement starts to decrease after this peak, posts with more than nine hashtags still have higher engagement than posts with fewer than eight hashtags, the report adds.
"This suggests that on Instagram, it's always better to err on the side of more hashtags rather than less," the report says.