i keep reading these articles about twitter where some old-timey news person says they don't "get" twitter. their complaint is usually, "who cares what you're eating for breakfast? why would anyone want to read that?" and i completely agree. this is a stupid reason to use twitter. people read the "what are you doing?" question on the twitter home screen and feel compelled to answer it, even if what they're doing is unimportant to anyone. i say this as someone who has maintained a public weblog for 10 years and posted a lot of unimportant things.
i think twitter's strength is not at all what its creators had in mind. while you can use it to keep track of what your friends are doing, its public nature makes it uncomfortable to use that way. and unless you're shaquille o'neal, the hundreds of internet strangers following you don't need to know what bar you're headed to tonight unless you hope to have them all join you. private services like loopt that integrate with phones and location services are better suited for that.
twitter is realtime enough that you can respond (with some real action or just a twitter reply) to current happenings, but archived well enough that you can use it to research topics and get opinions on things. this makes it a newer lazyweb that you can use to ask questions and usually get immediate responses from the people that follow you, but also get replies hours or days later from strangers searching for the same topic.
a lot of companies like dell are using twitter to respond to customers (so you better squat on your account now). their customers get instant gratification by having a question or concern addressed in a matter of minutes, while the company gets to show off their great customer service and have an easy (read: free) way of marketing to the hundreds or thousands of customers that may follow them.
if you are a company marketing your twitter account as an official contact point for your customers, treat it like your phone or e-mail. respond in a timely fashion, provide a professional but personal response, engage your customers, and ask them for feedback. don't just regurgitate your product site's rss feed. if your customers care enough about your company to follow your every update on twitter, they are probably subscribed to your rss feed anyway.
also, use the rss feeds on the twitter search page to see every twitter update mentioning your company and products. unless of course your company name is superblock and your products are corduroy and velvet, in which case you'll have to read dozens of stupid messages every day that have nothing to do with you.
i personally don't post on twitter very often. i use it for short messages that aren't worth typing a whole notaweblog entry for, or something that i want a wider audience to see. i occasionally get emails from twitter telling me someone new is following me but upon visiting that person's page to read their last few posts, more often than not, the updates are devoid of any meaningful content and often posted way too frequently. i don't want to follow that person and have them drown out the meaningful messages i like seeing from the other users i do follow.
so before posting on twitter, my rule of thumb is: ask yourself whether you expect anyone to respond. if you were talking to those same people on the phone or writing them an e-mail, you wouldn't keep saying things that they wouldn't respond to. you'd hopefully say things to engage them in a conversation or ask them questions.