Twitter is different than plumbing, sewage, currency, etc. because it was not created in response to an urgent need of sustaining life. While it might not have been clear at its inception, Twitter has changed how we thing about public discourse and politics. In the case of simple technologies, the needs of society drove the creation, implementation, and use of such things. Now, we have Twitter, the existence of which has changed our society in ways that Jack Dorsey & al. probably didn't intend.

The big idea from Postman on this is that with simple technologies, the culture and the society drive their development and use. Whereas with bigger technologies (Internet, statistics, Twitter, etc.) start to shape society around them.

I'm doing a poor job of explaining myself; Postman has more to say on page 23 onward of Technopoly if you're curious for a better explaination.