Hashtag | WP
The pound sign [hashtag /oAnth] was adopted for use within IRC networks circa 1988 to label groups and topics. Channels or topics that are available across an entire IRC network are prefixed with a hash symbol # (as opposed to those local to a server, which use an ampersand ‘&’).
The use of the pound sign in IRC inspired Chris Messina to propose a similar system to be used on Twitter to tag topics of interest on the #microblogging network. He posted the first hashtag on Twitter:
How do you feel about using # (pound) for groups. As in #barcamp [msg]?
— Chris Messina, ("factoryjoe"), August 23, 2007
Messina’s suggestion to use the hashtag was not adopted by Twitter, but the practice took off after hashtags were widely used in tweets relating to the 2007 San Diego forest fires in Southern California.
According to Messina, he suggested use of the hashtag to make it easy for “lay” users to search for content and find specific relevant updates; they are for people who do not have the technological knowledge to navigate the site. Therefore, the hashtag “was created organically by Twitter users as a way to categorize messages." 
Internationally, the hashtag became a practice of writing style for Twitter posts during the 2009–2010 Iranian election protests; Twitter users inside and outside Iran used both English- and Persian-language hashtags in communications during the events.
The first published use of the term “hash tag” was in a blog post by Stowe Boyd, “Hash Tags = Twitter Groupings,” on August 26, 2007, according to lexicographer Ben Zimmer, chair of the American Dialect Society’s New Words Committee.
Beginning July 2, 2009, Twitter began to #hyperlink all hashtags in tweets to Twitter search results for the hashtagged word (and for the standard spelling of commonly misspelled words). In 2010, Twitter introduced “Trending Topics” on the Twitter front page, displaying hashtags that are rapidly becoming popular. Twitter has an algorithm to tackle attempts to spam the trending list and ensure that hashtags trend naturally.
Although the hashtag started out most popularly on Twitter as the main social media platform for this use, the use has extended to other social media sites including Instagram, Facebook, Flickr, Tumblr, and Google+.
On #microblogging or #social_networking sites, hashtags can be inserted anywhere within a sentence, either preceding it, following it as a postscript, or being included as a word within the sentence (e.g. “It is [hushtag]sunny today”).
The quantity of hashtags used in a post or tweet is just as important as the types of hashtags used. It is currently considered acceptable to tag a post once when contributing to a specific conversation. Two hashtags are considered acceptable when adding a location to the conversation. Three hashtags are seen by some as the “absolute maximum”, and any contribution exceeding this risks “raising the ire of the community.”
As well as frustrating other users, the misuse of hashtags can lead to account suspensions. Twitter warns that adding hashtags to unrelated tweets, or repeated use of the same hashtag without adding to a conversation, could cause an account to be filtered from search, or even suspended.