The Several Habits of Wildly Successful Twitter Users » Slacker Manager
If you just want to send a note directly to someone, you can get them via the direct messages web interface.

Damn ! I’m a twitter user myself, but how could I have overlooked that “direct message” feature ? Now you can think of twitter as a unified messaging interface. If the personne you’re sending a message didn’t configure anything then the message will wait to be read on the website. If she configured IM and cell phone, then she’ll get it on the IM when online, on the cell phone when not. Pretty cool !

The rest of the quoted article is worth to be read !

My old post about twitter