https://en.wikipedia.org/wiki/Hashtag#Origin_and_acceptance

I still remember, in the late 2000s and early 2010s, finding that somewhat weird too. I was already regularly using the Internet (including forums) well before hashtags were invented and when I started to see hashtags in all kinds of contexts, I on the one hand found it great that the Internet was apparently arriving in more people’s lives, and on the other hand somewhat disappointing that they weren’t using forums or wikis or anything like that that I was already highly familiar with, but this weird new thing called Twitter… oh well…