Yes, you can ... forever ...
You can use US postage since origin US but since int'l will take more than 1 stamp
Yes, you can use a Forever stamp on a postcard, but it's overpaying for postage.
As of 2016, one standard forever stamp costs 49 cents.
Yes. that is their purpose, to still be valid.
The word forever means forever. Theoretically, if the US remains in existence for several hundred years without interruption and the Postal Service continues under the US government, your stamp purchased in 2011 should be honored.
Forever
No, it is not a Forever Stamp.
Yes, you can still use a Forever stamp issued by the United States Postal Service in 2009. This was the revolutionary idea of the Forever stamp - you can purchase a first-class stamp at today's price then not have to purchase additional postage later when the price goes up. The stamp is called the "Forever Stamp" because it will be valid forever for a first class envelope.
A Forever stamp is good for one ounce of First Class Postage inside the US.
You have to use US stamps in the United States, regardless of the destination.
No. The forever stamp is intended for domestic use only and doesn't meet the requirements of the Universal Postal Union for international mail.
The value of the Forever Stamp is the domestic First-Class Mail letter price in effect on the day of use.