One of the great things about twitter is how quickly you can reach a wide audience.
The RT(retweet) is a good way to increase your message coverage instantly. If people like your message they will send it on to their followers and your circle therefore increases.
There are various free website that have really interesting statistics available on your twitter account.
They make it easy to see how many people your tweets have reached in total, by retweeting. You can also use them to check on the reach of your friends as well.