In-game Channel:
- According to my experience, you can recruit sprout at 3 city state by talking one by one.
- You may experience that member of some FCs shout in town for recruiting member.
- Some FCs post in Party Finder too.

In my opinion...
If Community Team of Western FFXIV can help, it'll be good too!
For an example, I saw @FF_XIV_JP retweeted message of FFXIV player with #うちのFCにおいでよ last month.
They helped on Public Relations of some Free companies.
As I know, some FCs recruited new member because of that.
I think it's good idea. At present, that hashtag still alive in Japanese Twitter User Community.　

I think Community Team should do the same too. By the way, Reddit is good channel too as Penthea said.