Who is your target demographic? That makes a big difference when it comes to which website will be the most beneficial. To give a basic idea, Facebook is going to be your best bet for an older crowd. Twitter and Youtube definitely have a younger audience, and if that is not who you are trying to market to, your efforts are going to be wasted. Also, there is a high chance that your ads will be blocked on youtube by anybody using Adblock. With Facebook and Twitter, you can take advantage of trending topics/hashtags to promote your work without getting blocked by the extension. Hope this helped!