The copyright on Twitter's maintenance page is still set to 2012.
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 this page since 2012 (totally reasonable), I think it should be 2012.
This blog post (http://www.photoattorney.com/qa-what-year-should-i-use-for-m...) also mentions that you can use a range for when it covers multiple years of publication (like a blog).
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.
That's what most people used JavaScript for before AJAX...