I've had to pretty much rebuild my fc with a few members who always remained until new a few times already. We usually recruit until we have a nice comfortable amount of people that are all social and get along well, but you can't really do anything to keep them from leaving, be it leaving the game because there simply is no content worth playing for them or they found something that suits them better.
I run a social / roleplay fc, so the content drought is not really a problem for the roleplayers. But even the non-roleplaying members sometimes just log on to see what's up. And since we're all pretty casual, we can pretty much always go to pony / bird farms where perfect groups of 8 are not an issue, since barely anyone even has any birds. Still, people will leave nonetheless, and the way my group of lovely nerds who stayed since back when we all came together cope with that is, just accept it, recognise it, and don't give up on rebuilding again and again. Because even if they all left, we still had fun teasing the brown haired hyur who was running after everyone's catgirl, or progressing our airship with the lalafell who was always so polite, or helping the ambitious highlander progress in endgame, or play triple triad with the miqo'te man who loved pirates.