How do I force my MacBook Air into "hibernation" mode?
I had simmilar issue and it was because my Macbook Air was connected to charger. When I disconnected it before closing lid - Mac went to hibernation as supposed.

Then I've read on somewhere ( forgot source ) that Mac won't go into hibernation mode until you plug out all external devices ( USB etc. ) and disconnect power adaptor. When it's connected to power it won't go to hibernate under any circumstantes ( so it's better to shut it down if you have to charge it ).