Thank you pinkunicorn and geoGRAV for your answer but I already know this stat, that's why I specified "(not juste the 366 calendar days, which already exist)". I apologize if I have not been very clear.
To be as clear as possible let's take an example : If you made a FTF on March, 14th 2013 and another one in March, 14th 2014, in the "Finds by find date" it will count for only one date : March, 14th. In my request it will count for two different dates so you may really see how many different days a FTF hunter was in activity since he start geocaching.
With this feature the ranking would be a little bit different and it would put more value in those who do not hesitate to often go out for a single FTF compared to those who register tens of FTF in one day during an event or by openin a power-trail.
I think it would be a complementary ranking to those that already exist.