The symbol many people use to know as a number sign is now referred to as a hashtag. On social media networks like twitter and instagram you can use hashtags to put your posts with other people that have used the same hashtag. This is a way of bringing people together who have similar interests or are at least posting about the same topic. You can now see even Wikipedia has a definition, which you can get to by clicking here.
My visual map explains the use of a number sign (definition here) as it was originally intended in the top left hand corner, in the bottom right hand corner it is talking about it in the context of social media and being used as a hashtag. It is surprising how the use of the same symbol can be used in so many different ways over the course of time.
Here’s some funny stuff about hashtags.