All good advice. I’d add (even though you implied it) answering ALL relevant/reasonable @ replies to you, the sooner after one was posted the better.

Here’s why: If you take into account ideas from behaviorism such as reinforcement of behaviors and extinction of behaviors, then you understand that nothing will be less reinforcing and more extinguishing than providing ZERO feedback to the sender of a message (spefically to you) or “taker” of an action.

They do something, including contacting you, retweeting you, and then… nothing happens. The likelihood of the behavior being repeated just went down. Time is also a factor, as in, if it happens hours later, the felt reinforcement will be somewhat lessened.

This is also why the Thank You’s mentioned under 4) are so important: Other than “merely” being a sign of politeness, they also serve the important function of making the behavior more likely the next time, of reinforcing it by means of a social “reward” gesture.

Add to that the reality that, on Twitter, beyond the circle of your own followers “you live” only in the @ replies of others (i.e. else no one new sees your Twitter username) , and it makes it doubly relevant to thank people for retweeting, #followfriday recommendations, etc.

You are in a sense repaying them with having their name (along with the social gesture) being seen by your followers. Social gestures in general obviously go a long way in SOCIAL media.

(One small kink here is the fact that many Twitter users don’t know about and therefore never change the default setting of “@ replies to the people I’m following” and not see these @ replies of you to others. I highly recommend changing the “Setting > Notices” to “all @ replies”.)

So, now you have additional good/scientific reasons beyond “merely” being social, polite, and a good “Twitizen”, for doing what’s right on Twitter.

Obviously it may not be possible for people with massive follower/friend lists such as (minor) celebrities to answer every @ and thank for every action, but the spirit of the above points still applies nonetheless. Brands especially should take careful notice.