A "carte de visite" doesn't meet USPS postcard dimensions. Essentially it's a 6x9 contact print. If it was done on a 4x5 paper, then it could be mailed as a postcard.
Now, how do you figure that a carte de visite could beat Instagram? (Remember: the current year is 2012, not 1850. In the intervening 172 years, people have become rather blase about photographs.)