Hashtags are tags that categorise a post under a topic and make it discoverable to people following that topic. Their importance differs between networks and has declined over time, today, most of the reach is determined by the content itself, not by the number of tags. They make sense in three cases: for narrowly defined professional topics with an existing follower community, for local tags, and for a company's own branded hashtag, which serves to gather content around a campaign or event. It is worth using a few relevant ones rather than dozens of generic ones that merely clutter the post. It is useful to check what is already being published under a given tag, so that the brand does not end up in an inappropriate context.
See also: Social network as a search engine, Brand safety, Organic reach.
