John Piper has written a post as to why he's choosing to use Twitter (@pastorjohn). It's very interesting & a huge challenge to all of us who use media & especially the internet, here's a quote to entice you to read his reasons:
"Now what about Twitter? I find Twitter to be a kind of taunt: “Okay, truth-lover, see what you can do with 140 characters! You say your mission is to spread a passion for the supremacy of God in all things! Well, this is one of those ‘all things.’ Can you magnify Christ with this thimble-full of letters?”"
Read the whole post