Excerpt from Mashable:
“The hashtag’s widespread use began with Twitter but has extended to other social media platforms. In 2007, developer Chris Messina proposed, in a tweet, that Twitter begin grouping topics using the hash symbol. Twitter initially rejected the idea. But in October 2007, citizen journalists began using the hashtag #SanDiegoFire, at Messina’s suggestion, to tweet updates on a series of forest fires in San Diego. The practice of hashtagging took off; now users and brands employ hashtags to cover serious political events (#Cairo) and entertainment topics (#MileyCyrus) alike.”
For some advice on how to use the hashtag effectively on your social sites, have a read of our article “All you need is love … and a few good hashtags”.