I've put $25 into Facebook and Adwords/YouTube. When I put the cash into YouTube I had an enormous growth month, on top of my usual SEO and Collaboration Videos/Guest Uploads. Facebook granted me a lot of promotion with a boosted post. I got likes out the wazoo, and 21 new followers on the page. However, the video i boosted only got like 42 extra views. Whereas my Youtube promotion would net hundreds more views and probably doubled my subscribers from the growth the previous month. Ultimately, I'd say YouTube is actually the biggest and best Social Media site for growing your channel to be frank. Also Ad Words gave me a $100 credit, whereas Facebook only gave me a $30 credit, so I'm biased lol.
So if we're talking promoting? YouTube > Facebook.