2007 Aug 25, Stowe Boyd, tweet, https://twitter.com/stoweboyd/status/226570552
I support the hash tag convention: http://tiny url.com/2qttlb hashtag factoryjoe twitter
2009, Paul McFedries, Pete Cashmore, Twitter Tips, Tricks, and Tweets
You can also search for a hashtag by typing a topic (without the ) in the search box and clicking Search.
2009, Alistair Croll, Sean Power, Complete Web Monitoring
While hashtags aren't formally part of Twitter, some clients, such as Tweetdeck, will persist hashtags across replies to create a sort of message threading.
2011, Rory Stewart, "Here we go again", London Review of Books, 33.VII:
The planes are moving into position. The foreign ministers of minor Arab states are taking calls on their cell-phones from Western politicians. Twitter accounts explode around the Libyan hash-tag.