The copyright on Twitter's maintenance page is still set to 2012. http://imgur.com/eb3AbX6
As far as I know, it doesn't have to (shouldn't?) be updated for the current year if it doesn't change. From http://www.copyright.gov/title17/92chap4.html#401 : If a notice appears on the copies, it shall consist of the following three elements: (1) the symbol © (the letter C in a circle), or the word “Copyright”, or the abbreviation “Copr.”; and (2) the year of first publication of the work If they haven't touched t…
Adding any copyright notice is optional since 1989 in the US, the year they joined the international agreement of the Berne Convention which was ratified in 1886 already.
The main purpose today is to demonstrate the owner claims the rights and is actually willing to defend them (in court), but it's not necessary to put it on any work.