As with anything to do with social media, I already feel this guide, just days old, needs editing, so please help me improve it by posting comments below or e-mailing me, sreetips at sree.net.
One edit I would make to the card (apart from perhaps removing my Twitter profile photo) is adding a line about starting tweets with an @ sign at the start of a tweet. If you do so, it will only be seen by those who follow you and the person your are mentioning--usually a small subset of your followers. If that's not your intention then start your tweets with a word or even a period (".@sree is a lousy tweeter..."--without the period, only those who follow me and follow you would see it). I would therefore also change the first line under "@username message" from "public reply to a user" to "semi-public reply to a user," emphasizing that you are losing potential readers by tweeting that way. If all you meant to do was reply semi-publicly to another person, then that works fine, of course.
Meanwhile, my more detailed, constantly-updated Social Media Guide is over on Tumblr.