This is all good advice but realistically you can probably skip the random social media sites and just do email and YouTube. Email is much, much better than pretty much any social network.
Somehow you will need to reach people, at least initially, though. They don't magically appear on your mailing list.
Mail lists go far, but retweets go wide. Different tools for different purposes.
Email? Outside of the older crowd, I don't know anyone that actively uses email for anything. So many people in my life are surprised when my wife and I use email for anything. Tech/business/academic might hit some ok % with email, but outside of that I'd doubt you'd get to 1% of your potential audience.