I must say that the new expiry policy that distinguishes between miles acquired from flying and other miles makes things rather complicated. My partner's last EQM/XP-earning flight was in Spring 17. Miles should therefore normally expire in Spring 19. He has, however, earned FB miles through car hire in Autumn 18. His online account gives the expiry date of his miles as two years from the car hire transaction (i.e. Autumn 20) rather than 2 years from the last EQM/XP-earning activity.
If he were to rely on what his account says, he would be OK not flying until 2020. I have a strong suspicion, however, that, if he does that, he will lose all of his miles except from the handful of miles earned through car hire.

I tried to go and double-check what the FB TandCs say but I am afraid that they are even more ambiguous, confusing and erroneous.
They define to "overall extending activity" and "partial extending activity" but the concept of "partial extending activity" is not used at all in the rest of the T&Cs, making it an empty concept.
The TandCs state that the validity of all miles will be extended for 2 years after the last "overall extending activity", defined as an XP-earning activity or "any other Earn Activity designated as an Overall Extending Activity in the Flying Blue Communication" (and they give Transavia flights and FB credit card payments as examples of the latter)..
They do not however state which activities and for how long are subject to "partial extending activity" other than stating that only miles earned after the last overall extending activity can be partially extended.

I think I understand what they intend to do and why with "partially extending activity" but the TandCs do not reflect that. This is just one big confusing mess.