Should a text message be treated like a letter or any other form of communication?

Yes- If a dialogue has been established, then it's rude to leave the other party "on hold" too long by not saying "let me hit ya later" "BRB" or something to let the other party know you're busy and will continue later OR to "hang up in his/her face" by N

No - It's just a text message and shouldn't be placed into the "communications" category at all...