I think it's a good idea, it can be a quick link to search anything in newsfeed archieve which similar to the words with hashtag
and also mentions, sometimes we need to reply our replied status to many different persons, and mentions can be use as a quick link to repliers profile :)
I agree with both of you guys.. or perhaps for first starter, let the "mentions" (@) first being applied, it would be great and on another next version hastag or url link generator could be useful too.
Maybe even better with the addition of a trending list when users use a Hashtag(#) and then they can see what is trending on the index, dashboard, and sidebar.
Sounds excellent, take ya time and drink lots of coffee, better to really get it right. So far you have come leaps and bounds on your idea. The fact someone is even making this and is showing consistent steps forwards in the development is great news for me as im sure for many others.