I recently had a chat with a social media specialist, and my business coach, and here is the gist of what I got from them -
Facebook is #1 for promoting a YouTube channel, at least at first. To build a fan base, you need to start with friends and family. These people want to support you, and word of mouth from trusted friends is more powerful than anything else. My coach told me to ask all of my friends to also share every post I have on Facebook, because again, that way trusted friends are giving a personal recommendation to their friends and family. My coach also told me to go add a bunch of people on Facebook as well. Anybody I have mutual connections with.
Instagram is my #2. I won't say it's better than Snapchat because I'm not on that platform, but from what I've herd from some people younger than me, Instagram "stories" (not your regular feed) are kind of making snapchat obsolete, because it's sort of the same thing in many ways. I can't say for certain how much traffic i've gotten from Instagram, but if you set up your account as a business page, you can see your analytics and how many people clicked on your links, checked out your profile, etc. I think it's especially good for certain types of channels. I have a food channel, so posting pics of food is important for my brand, which makes Instagram a good fit.
Twitter is obviously a major platform, but the social media specialist told me he's not really too high on it anymore. Posts are quickly lost in peoples timelines, as there is so much info flooding into Twitter every minute. I'm no expert on any of this, but I have definitely gotten the least amount of action on Twitter. It's great if you want to to voice your opinions frequently so you can stay in peoples feeds, but for me, I don't like just posting my thoughts all day long. I do post to Twitter when I have a video or an image, but I don't prioritize it.
Again, I'm no expert, I don't have a whole lot of subscribers on YouTube yet, but this is where I'm at with everything. I imagine things change as your following grows. If you have some cash to invest, you can also get someone to build your social media followings for you, which I have not tried.