I must respectfully disagree that FC status was given to Delta's most loyal customers.
I was seated next to a guy a few years back when he started bragging about being a FC. He told me that it was his first time to ever fly Delta, that a friend of his who worked at the Delta marketing office in ATL had sent him the card.
Here I was flying over 100 segments per year on Delta for my free Crown Room membership, and a guy is bragging that he may fly three or four trips per year on Delta, but had been given FC status.
When I returned home I went to my local Delta marketing office and asked about the FC program. I was told the program was to be discontinued in the future, which it has been. I was also told that FC's were people which Delta felt could steer business their way....and it had nothing to do with how much a person flew Delta, it was strickly a marketing program.