IAC owns a number of websites you may recognize, including Match.com, OkCupid, CollegeHumor, and Urbanspoon. Justine Sacco is (was) a public relations representative for the company, so you'd think she'd know better. Apparently, she doesn't.
The company had this to say about the incident:
This is an outrageous, offensive comment that does not reflect the views and values of IAC. Unfortunately, the employee in question is unreachable on an international flight, but this is a very serious matter and we are taking appropriate action.