Great discussion!
I've been doing community forums / online discussion groups for a LONG LONG time! I've realized that attrition, especially in a topic-specific niche is very common. People leave online forums for TONS of reasons, for example: I used to be extremely active and passionate in a Hyundai Elantra online group, but over the years I lost interest in working on the car and thus also in the community.
BYC is, IMO, a little bit unique in the fact that many people come for chicken-specific information (often called the "interest graph") but then build friendships and hang aaround with their friends to chat, etc. (the "social graph"). This tends to help keep people active longer, but still isn't a guarantee that they will stick around indefinitely.
We love when the old-timers come back to visit. We have lots of great real friendships that develop over time!