I'm not sure whether I'm more admiring or envious.  Since joining Twitter, I aim to tweet something of my own at least every couple of days.  Often it's a struggle.  I'm constantly amazed at how some people come up with not one or two tweets a day, but more than a hedgeful of sparrows.  Where do all the ideas come from?  How do they find the time?  Do they do anything else?  Why do they do it?  There are some people (myself included on occasion) who tweet to draw attention to their other work.  For others, it's obviously primarily for fun, like doing a crossword.  For the majority, I suspect it's mostly to communicate something they feel or have observed that would otherwise be lost, or to share how they see the world without getting on a soapbox.  
Of course, quantity doesn't necessarily mean quality, but many people create one exquisite gem after another, and have the ability to move you with a phrase.  The most wonderful thing about Twitter is that while you're struggling to write your own miniature masterpieces, you can share all your favourites by those sickeningly prolific tweeters at the click of a mouse.