It's not just who's on there. It's also how the platforms promote content into your feed. When I was on Facebook in 2008 the friend feed was just that. Just people I mutually knew IRL posting. Facebook hadn't yet figured out how to really monetize it. Advertisers were not as on it. SEO wasn't really a thing yet.

Fast-forward 5-6 years and it really grew into an all-encompassing thing. Yeah, more people were on it, but so were the marketable opportunities. So were the suggested posts. So were all the news organizations, the grifters, the advertisers... and Facebook's role in all of that is to promote the most outrageous and engaging content to you to keep you on the site longer than ever before. They have it down to a science.