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.
I can log in, but I'm getting a 503 with the following: "Something is technically wrong.
Thanks for noticing—we're going to fix it up and have things back to normal soon."
Back up, and they sent out a message on status.twitter.com:
Update [12:12pm PDT]: During a planned deploy in one of our core services, we experienced unexpected complications that made Twitter unavailable for many users starting at 11:01am. We rolled back the change as soon as we identified the issue and began a controlled recovery to ensure stability of other parts of the service. The site was fully recovered by 11:47am PDT. We apologize for the inconvenience.
There will be a lot of graphs that don't look great around the operations team. I wonder how much of an impact each minute that the service is down has on their uptime %.
You say that sarcastically, but I imagine that many brands, who have advertising campaigns scheduled for one of the more lucrative days of the week, will be less amused.
I'm not necessarily referring to Sponsored Tweets; brands often coordinate word-of-mouth campaigns across all forms of media too, and Twitter's downtime would hurt that too.
http://imgur.com/eb3AbX6