http://en.wikipedia.org/wiki/Telecommunications_data_retenti...
It's a really complicated legal issue, and the last thing you would want to do as an international company is shoot yourself in the foot by deleting "evidence".
"The United States does not have any Internet Service Provider (ISP) data retention laws similar to the European Data Retention Directive. All attempts have failed:"
THEREFORE: Under what law was FB required to retain the data, which was eventually handed over under subpoena?
But I think i remember something about Facebook having european offices in Ireland, so for us Facebook users in the European Union it's the Irish and EU laws they have to abide.
But I havent got a clue about what law regulates theese things.
http://en.wikipedia.org/wiki/Double_Irish_arrangement
The location of its legal tax headquarters is up for debate; Additionally, Facebook has corporate offices all over the world in different countries: https://www.facebook.com/careers/locations Italy, India, Canada, New Zealand, Belgium, etc.
Clearly it's not just a "US" company and only has to follow "US" laws.