First up is Twitter. Find hashtags applicable to your content, and tag your post accordingly with a short link to your video. Any character space remaining can be used as a title/description to catch people’s interest. With the right tagging and words, you'll find views easily.
Tumblr, like Twitter, is mostly tag-based. However, the tags you can use on Tumblr are nigh unlimited- feel free to go crazy with all the tags you’d like your video to appear in. Tumblr posts have the room for a full title and description, which you’re welcome to copy over from YouTube. Adding extra details or a link to your Tumblr blog is something you can do to add to engagement, and posting special content on your Tumblr will add value to the platform for your fans.
Facebook functioned before tags, but now it, too, uses them. With Facebook tagging, you’re best-off using the single most relevant tag that pertains to your video. You’ll have room for your URL and whatever you want to add, and FB offers you the ability to post to a fan page. Using fan pages will put all your content in one place on Facebook, and give your audience a place to interact directly with you.
Don't use reddit, unless you're trying to promote well made* content.