I always put a quick little something on my Facebook when I have an opening! I never do it like an ad, more like, hey friends, if you know anyone looking I have a space open. I don't get into any details or anything, just short and sweet.
I get sooo many referrals from Facebook friends who see that I have a spot open and let their sister, neighbor, coworker, etc. know. I almost always fill right back up after posting. Actually right now, all my dck's are from some form of Facebook advertising
Good luck!!