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 .
The trending topics can be organized by geographic area or by all of Twitter.
Hashtags are neither registered nor controlled by any one user or group of users.
Twitter uses a different syntax for Chinese characters and orthographies with similar spacing conventions: the hashtag contains unspaced characters, separated from preceding and following text by spaces (e.g.
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.
("Pound sign" in the UK means "£"; "#" is called hash, gate, and occasionally octothorpe.) In 1970, for example, the number sign was used to denote immediate address mode in the assembly language of the PDP-11 In the 1986 SGML standard, ISO 8886 (q.v.), # is a reserved name indicator (rni) which precedes keyword syntactic literals, --e.g., the primitive content token #PCDATA, used for parsed character data.
The pound sign was adopted for use within IRC networks circa 1988 to label groups and topics.
A hashtag is a type of metadata tag used on social network and microblogging services, allowing users to apply dynamic, user-generated tagging that makes it possible for others to easily find messages with a specific theme or content; it allows easy, informal markup of folk taxonomy without need of any formal taxonomy or markup language.
Users create and use hashtags by placing the number sign or pound sign (also known as the hash character) in front of a string of alphanumeric characters, usually a word or unspaced phrase, in or at the end of a message.