Here’s his rational rationale:

I see two kinds of response to social Internet media like blogging, MySpace, Facebook, Twitter, and others.
One says: These media tend to shorten attention spans, weaken discursive reasoning, lure people away from Scripture and prayer, disembody relationships, feed the fires of narcissism, cater to the craving for attention, fill the world with drivel, shrink the soul’s capacity for greatness, and make us second-handers who comment on life when we ought to be living it. So boycott them and write books (not blogs) about the problem.
The other response says: Yes, there is truth in all of that, but instead of boycotting, try to fill these media with as much provocative, reasonable, Bible-saturated, prayerful, relational, Christ-exalting, truth-driven, serious, creative pointers to true greatness as you can.

The fact that he limited the responses says a lot about how he’s using Twitter. The third response is that it’s a great way to see how people who have similar interests to you are thinking and it’s a great why to share information and links from around the Internet. It’s also a way for people you know to comment on stuff that your both interested in, in other words, it’s a “social internet media.” That part seems to be missing from Piper’s use – as of this post, he’s following no one. He might as well blog, preach from the pulpit or write a book because that’s how he’s using the medium. He’s proclaiming (for our edification of course, so that we read his tweets instead of looking at porn — of course I had no intention of looking at porn) instead of engaging in a dialogue (for this reason Mark Traphagen called him the evangelical Oprah :-).
I’m following him of course. His tweets are at least interesting, unlike some of the other well-known Christian “celebrities.” You can follow Piper, here.
(Thanks to Mark Traphagen for the heads up)