I finally signed up for Twitter, though I'm not entirely sure why. I'm following on Twitter all of the people I have in my feed reader; I have a few of them as IM contacts; and I see most of them in IRC #habari most of the day, too. The best analogy for Twitter that I can think of is "asynchronous public messaging". It can be used like instant messaging, where you chat back and forth with contacts, except that it's available for everyone to see and it doesn't happen in realtime.
Some folks use Twitter to post short updates about something that they want to share that might not merit a full blog post. Other folks use Twitter to ask the same question of a group of people. For example, ringmaster asks Philly folks about where to dine: rather than send the same IM or email to all of his contacts, he can put it on Twitter and the folks who follow his Twitter account can reply as they see fit: IM, email, Twitter response, etc. Ringmaster's use of twitter is one of the best that I've seen: he has several groups of online acquaintances and he communicates with them quickly and succinctly through Twitter. I'm not sure that I'll use it nearly as aggressively or effectively, but I'll give it a shot.
Looking around, I see that there are tons of similar ways to support ad-hoc communication for various purposes. In addition to Twitter, there's Tumblr. Some folks use Flickr for quick group chats. Then there are the bigger institutions like Facebook, MySpace, LiveJournal, etc.
I have a blog, a Flickr account, and now a Twitter account. I'm also on several IM networks. I'm left scratching my head at why I should pursue any of these other channels for connecting with my friends. I'm not interested in meeting new people, particularly, and I'm not a compulsive signer-upper for new online services. What do these new services offer me -- the user -- that is of substantial value over what I already have?
Perhaps I'm overly skeptical: I recognize that most of these online services are commercial ventures, and their driving motivator is to earn money. Some are more commercial than others; some aren't yet as commercial as they're likely to become. Are they truly offering me a new, valuable service, or are they merely offering me something in order to make money off of me, whether that's through paid upgrades, or my eyeballs on their partner's advertisements?