Many companies have begun using Twitter, the microblogging service, to send people brief messages and communicate with customers about new products or how to improve their services.

The pool of users to communicate with is increasing. Twitter doesn't release numbers since it's a private company, but Compete, a web analytics company, estimates that Twitter saw at least 2.5 million unique visitors in August, up 12 percent from July, and a five-fold increase over a year ago.

Because a Twitter message (known as a Tweet) must be sent in 140 characters or less, you need to get to the point of your message quickly and it must be succinct. Most of all, according to Laura Fitton (who goes by the Twitter handle @pistachioand who runs a consultancy that helps companies utilize microblogging), you need to be interesting.

"Don't be boring and don't be selfish," she says. "More than other platforms, people want to hear what you're saying, but it still needs to be engaging."

And by selfish, she means don't act like an annoying product pusher. Here, we take a look at four ways companies and their employees are using Twitter to engage customers and what kind of messages they're trading with them to draw them to their products..

1. (Careful) Product Pushing

As a company, you should avoid being blatantly self-promotional on Twitter, Fitton says. Because people are choosing to subscribe to your site, however, she says you can assume they'll expect (and want) some discussion and updates about your products. But the key is to have some voice or commentary in your tweets to go along side product information.

Messages targeting customer problems and needs are more useful than blatant advertising pitches.

Jetblue (@JetBlue) has been especially effective at striking this tone. Here is an example of one of their latest tweets:

"JetBlue has landed on eBay. We wanted to try something different. We're auctioning off some great packages on eBay http://jetblue.com/ebay/."

Kodak, which has a chief blogger (@kodakCB), wrote about how her company's product was being used rather than giving blatant product pitch:

"Spent the weekend at the Toronto International Film Festival. Saw some great movies shot on Kodak film!"

The messages suggest this lesson: Avoid marketing speak and be up front and honest. JetBlue and Kodak's tone is informative, sometimes nonchalant, and less pushy than, say, "Come check out all our great deals on eBay!" or "Go buy our newest film!"