I rather let my videos be found organically using good SEO, good keywords, good titles and good thumbnails.
The only other places I have promoted are on my own profiles on twitter. Promoting anywhere else can often get you seen as a spammer which will do more harm to your channel than good. Asking strangers to watch your video when you don't even know what they're interested in, is less effective than simply making sure your content is discoverable for people who ARE looking for your kind of content.