I was put onto this interesting feature of Twitter by Scott Hanselman, where needs repeating. When you use Twitter to send a tweet all of your followers will get it, unless when you start the tweet with a name (called an @ reply). For example:
The above tweet would have been seen by everyone who follows me!
This second tweet starts with William Branders name, so only people who follow BOTH of us will see it. In other words, it is only that group in the middle of the following Venn chart which will see that tweet!
So how can you send a tweet starting with someone's name & have all of your followers see it? Start the tweet with a dot, like in the example below.
A final important to note about starting a tweet with a name, those tweets are still public. Even though your followers don't automatically see them, they could just browse to the Twitter website and find them their. If you want a tweet that is private, you a direct message