Adding www Doesn't make any sense for URL shorteners for example. The same occurs on media like Twitter where chars are counted and "precious": using www. adds 4 chars to the message (in theory, since those url shorteners are in help).

Another detail I've noticed since wide adoption of browsers that include a single combined url/search entry field. Most people don't even care about the exact URL, they just enter the name they believe the website is, and let the search engine do the job if mistyped or inexistant. (That leads to phishing attacks).