Like SMS, email, the fax, phones, the telegraph and the spoken word before it, Twitter is now a communication platform.
Yesterday, while sailing on a J/24 I unfortunately nailed the skipper’s head with my own. Or course, I shook it off and continued racing. It wasn’t until later, while playing tennis that my vision was going blurry and I was quite dizzy. Having been through a concussion a few years back, I simply didn’t want to take any risks.
Which brings me back to my main point. I Tweeted that I was heading to the ER and that I had suffered a concussion (sorry about the spelling!). I did this after not being able to reach my wife. Upon tweeting both messages, my sister picked up my status on Facebook (which picks up all my tweets) and contacted my wife as well and my parents.
Twitter and Facebook both become a clear channel for information and action. As not less than 30 minutes later did my Father show up at the Hospital as well. He had just arrived back in Seattle after a business trip in the Midwest and was able to learn about what was happening, act on it, and help both my wife and I out after I was discharged.
Twitter is simply no longer a novelty. My friends and family were notified and acted upon what I Tweeted.
Lastly, I would suggest that Twitter is more of a communication platform and that Facebook is a communication system. Facebook facilitates communications between people that already know one another. Twitter is open to all who wish to follow and interact. I had people I do not know directly providing connections to help and services that I might use should I need them.

Glad you’re whole and healthy.
I think @Biz also articulated the hypothetical nicely in an interview with Maureen Dowd:
DOWD: I would rather be tied up to stakes in the Kalahari Desert, have honey poured over me and red ants eat out my eyes than open a Twitter account. Is there anything you can say to change my mind?
BIZ: Well, when you do find yourself in that position, you’re gonna want Twitter. You might want to type out the message “Help.”