Hashtags has been popular before Facebook and in a number of other media.
http://factoryjoe.com/blog/2007/08/25/groups-for-twitter-or-...
However I have a strong feeling that I modified/used hashtags (as tagging, not as a channel) back in 2005/6.
I think I got the idea from another system (probably some early wiki) but I'm unable to recall now.
I would guess it was inspired by IRC, the first popular chat system, dating back to the 70s or 80s, which uses channels named with the hashmark and which are often named after the conversation topic
https://en.m.wikipedia.org/wiki/Fragment_identifier#Basics
#Basics