I could remember using Hashtags at 2009 in my twitter account – #MyfirstHashtag. Then in Instagram, google +, Finally on Facebook too. Using this #Hashtag is awesome.
What Wikipedia says about hashtag? “A hashtag is a word or an unspaced phrase prefixed with the hash character, #, to form a label. It is a type of metadata tag. Words or phrases in messages on microblogging and social networking services such as Facebook, Google+, Instagram, Twitter, or VK may be tagged by entering # before them,”
Following are the Social Networks which uses Hash tags:
This below given Infographic says the History of #Hash Tags