Hi, hope you are doing well.
I am confused about the point "2. User is notified when one of the users on his list uses a hashtag (#EXAMPLE) he/she hasn't used before."
does he/she mean the user who is using the tool or the user in the list?
What types of tools are you expecting to build? ( native, web app, Chrome Extension etc)
Looking forward to discuss with you more.
Thanks
Apel Mahmod