I would suggest you set out clear goals for your FC. Such as Raiding, Events, Leveling, etc. The more interaction you have with your fellow FC-Mates the more likely they will stick around. However don't limit yourself to only your FC. Make use of Linkshells as well. This game has a lot of people who have old time friends they play with and are in the same FC so they don't want to leave, but they will seek out people for raiding or other events through linkshells instead.

My current FC has a handful of people on at any given time, and our numbers are quite small, but we have 2 separate raid groups for Coil, with various people from different FC's. However we all come together for the raid times and just have fun in general. We aren't hardcore, we'll spend 1 hour on Coil these days and then everyone else is free for the rest of the week, but we have found a nice group of people who are mature and easy going.

You will also want to find like-minded people for your particular needs or wants of your FC, and that will take some time.

As far as recruitment you can use forums (such as the server forums here), or Party Finder, or even /shouts. But if you just say you are an FC looking for members people will be reluctant to join. Why should they join? Are you raiding? Are you just leveling? Got to give people some incentive.