Howdy, everybody!
I was recently watching videos of VBACs on YouTube. Watching live births always gets me teary-eyed.
Anyway, after each video, I posted comments about the video and included the title of my book in my signature. Doing this resulted in an organization inviting me to speak in California.
Unfortunately, I had to turn them down (I don’t have time to travel to California), but YouTube is turning into a great way to promote your book, provided you truly do participate in the community…and don’t just spam them. 
And, believe me, you can find videos on just about any topic in the world on YouTube.