I've used Youtube videos in the past to promote my forums but I didn't get much traffic from them. Now that I am close to 400 subscribers on my main channel maybe it's time to look into promoting my current sites on my channel now that I have way more subscribers than I did when I tried in the past.
I do not use YouTube to currently promote my forum (which isn't open to the public yet) but I do think having some YouTube videos would help get more activity on the forum.