Is Twitter's tweet ID unique for all the users?

I think Twitter's tweet Id (Id assigned to individual tweet) is long. But as I was looking at the stats I found that twitter has around 177 million tweets in a month(2011, maybe much higher now). So by this all the unique combinations would not last long. So my question is 'is tweet Id unique for all the users or it is unique for individual users'. If it is unique for all the users how does twitter manages it?