Most traffic causing Facebook, Youtube, Twitter, and other such sites to appear so popular is through paid bots and artificial traffic. Those are used to astroturf and generate fake popularity.
Now I'm not saying there's no actual people using those sites. Heck even I use them. But the people who actually use the internet regularly is in the single-digit millions. A lot of them are on the chans, on Reddit and other "meme" websites, as well as a bunch of niche forums.
The Internet is a lot less populated than you think.