Yes it is. That is what they make their money off of, the difference between what they earn off of the labor of the video creators and the money they pay out. If nobody uploaded videos, youtube could not sell attention just like if no steelworkers arrived, the steel mill can not sell steel. That applies to all social media/user generated content platforms.

Now, you might argue that this does not feel like labor because it's more voluntary than labor usually is. Which is totally correct and exactly why this industry is so lucrative.