It depends on your goal. If your goal is to drive views to your YouTube channel then post an enticing clip of your video that makes the viewer want more and tell them to click a link to your video on YouTube.
This is fairly ineffective if you don't have an audience already on whatever platform you're posting on, however.
So then you get bogged down in trying to build a facebook or twitter following by using a bunch of hashtags...so that hopefully after you build that following and you post links, some of those followers will click links to your YouTube video... and you get completely burned out and go nowhere.
Your best bet if you're small is to make great videos that people want to watch and learn everything you can about YouTube's algorithm and make sure you're doing everything you can to get noticed by YT search and recommended videos. TubeBuddy is priceless for helping with this.
Even if someone has pretty much the same video with millions of views, your video could be recommended for further viewing.
Once your video views are rolling, the next thing you need to focus on is building an EMAIL LIST from your channel (I've written a guide about how to do this). Here's why that's the next step:
If you have 10,000 followers on facebook and you post a link, maybe 300 will click it
If you have 10,000 email subscribers and you send a link, probably more like 3,000 will click it
The reason is because of nOiSe. There's so much crap coming from every direction on social media you have slim chance of being noticed, plus you have to contend with each social platform's algorithm that will probably filter out your link that leads off that platform.
There's much less noise in email and you beat the algorithm automatically if you follow the legal rules of email marketing, which is easy.