And another update:
while in the past you could earn level miles with a French cobranded Air France/KLM Amex-card for every purchase, this was ended a while ago.

But from September 2013 when you buy from Air France/KLM you earn (only for those purchases) level miles. So when you book tickets for other people you can earn elite status without flying again, only it will be much more difficult then before.

The new rule is not only for the French cobranded card, but also for the Dutch one.
The Silver card does not earn level miles, the Gold card earns 1,5 mile per euro, the Platinum card earns 2 miles per euro.