I don't think Pinterest is a good forum for advertising unless you accompany your videos with a blog post - and if they're more tutorial-y (ie. DIY type things.) Tumblr is a good place - it's easy to use hashtags to get followers as long as you obviously hashtag appropriately. You can even go around and message people and reblog their own stuff to get a following - similar to twitter but you can make your blog look prettier

Reddit, I hear is a hit or miss. Lots of people get positive feedback but if no one likes your video then it'll be downvoted to oblivion. Plus I'm scared of the community there as they seem very forward in their language. I think you can try Google+! There's a fair amount of small communities for Youtubers and you can simply share your video and get a couple of views, have some chats with people and best of all - it's connected to your youtube account so it's simple!
I would suggest venturing into Tumblr - but even that is a hit or miss. I think Instagram, Twitter and Snapchat are your biggest leads in terms of social media presence.