Yes, it would be worth mentioning this activity. Not only have you earned money from this extra-curricular (putting it on par with what some high schoolers earn from part-time jobs like babysitting or tutoring), but you have a substantial following. One of the things that's extremely important when applying to selective schools is to show that you are passionate about something and are willing to put in the work and focus to make that thing successful. Admissions officers will see your 70K+ subscribers and 8 million-plus total views for your channel as a reflection of the hard work and time you've put into this activity, and will know that you're capable of putting in the hard work to make something succeed.
That said, if you do put this down on the activities section, you need to make sure you explain the significance. Don't just say "Content creator on YouTube;" instead, something more along the lines of "Creator of the 70,000+ subscriber, 8 million-plus total views YouTube Channel [channelname]" conveys more information about why it is admissions officers should care about this activity. It's possible that some admissions officers will know about YouTube Let's Plays and gaming videos, but numbers and data will clarify the extent of your accomplishment for those who don't.