I've been in many FC's prior, and most of them are very successful, and active even during no patch or only hotfix cycle. The reason being, they do not demand that everybody and their pony sits on any voice program aka discord/teamspeak/skype/mumble/whatever else ppl are using.
They also don't force their members to participate in casual content, that nobody got time for, or want to participate in (beach parties/chatting in their houses). Nor are members asked these hundred questions upon login "how are you doing/how was work etc."
During actual content update, these FC's striving to make sure, that everybody has their static spot. That primals and raids are only cleared and trained with FC members. Farm parties are set up etc.
That does make a huge difference in my eyes, plenty of times I've switched FC's because I do not want to hear all day, how somebody did their dishes and what they are planning for dinner, when being forced to join VoIp.
And before people start with this anti social non sense, it has nothing to do with it, but back in the day you could be social with friends and enjoy games without having to listen to their actual voice and daily problems.
Sadly nowadays, everybody thinks that sharing their regular activities must be posted on social network platforms, and pushed down ppl's throats on discord. Nobody got time for that. Maybe if some FC's would change that, they wouldn't end up having no members or barely activity during non patch cycles.