Since signing on to twitter, even I have seen an exponential growth of users... The reason for me is simple.. its simple! Unlike Facebook there are no 3,000 extra apps knocking at our door before you can even get to your account. Its very immediate and the best is you dont actually have go through the web interface.
There are numerous little applications, many of them AIR apps (Flash and Flex based) that sit in the background (More often in the foreground) of your computer loading in the tweets.
I've been trying out a few. I started with Twitterific http://iconfactory.com/software/twitterrific. I'm still using it occasionally, it has a clean simple interface, but some times I find it too simple. When I get into twittering with my Twfreinds I like to organise a little, list them in @replies or list the direct messages I have and for this I do like Twhirl ~ http://www.twhirl.org/. This is an AIR app and I used to find it sluggish, but I think they have streamlined the CP usage so its a lot more responsive. Very customisable. Finally I came across Skimmer ~ http://www.fallon.com/. This is great it encompasses all your socials... again it can be sluggish as is the nature of AIR apps sometimes but its very clean and groovy. Hasn't got the functionality of the Twhirl but I really like it, its beautiful and like beautiful objects we forgive a lot.
Thats touching on the subject of why beautiful things work better.. they do, but that's for another time.