Jenova looks like the least populated server, atleast right now it does. Malboro seems to have a good mix, more active during the evenings Eastern time. Depending where I am you'll see some conversation going on, or atleast start your own. Most of the time if you shout out someone will answer you, may not like the answer, but they're alive anyways.

Don't know much about the FC's as I am not in one. But there are plenty, always getting the random invite. Sometimes asked why I am not in a FC. So people will spark up conversation.