Depends entirely on your goals. I know some FCs shoot for a large population and so they throw invites to anyone and/or shout in towns that their FC is open for business. A FC I'm in an alliance with will shout on rare occasion when their population has taken a bit of a decline, but otherwise they recruit people they meet and/or people who show an interest.

My FC is/was always shooting for slow growth. A couple forum posts to make ourselves known was our advertising. Many of our newer members chose to join because of experiences they've had with our members either on forums or in game, so being present in chat in DF etc. I'd highly recommend, you're always representing you're FC. At one point we were growing too quickly for our own liking and had to stop recruiting for over a month, then when we relaunched we tightened up our application process a bit to discourage those who aren't willing to make a bit of effort and make sure those applying knew what they were applying to.