Jim (7165551212): Hey what's going on? #notdoingmuch
Joe (7165553637): Nothing much dude. So uhh what's that hashtag for?
Jim (7165551212): Oh I wanted to show you I'm not doing much
Joe (7165553637): Why didn't you just say that then?
According to Wikipedia, "a hashtags are words or phrases prefixed with the symbol #... and are mostly used as unmoderated ad-hoc discussion forums."
So if you use a hashtag on Twitter, you are trying to make your hashtag a trend, a phrase people can search. Short story long, a hashtag should not be used in a text message.