Communities. People want other people to talk about what you're talking about with you. You can add a comment section, but it requires mapping between commenters on your site and commenters on another guy's site in order to really have a comunity with repeated interactions between individuals. And of course if you're new it's hard to get traction on your own site without posting comments that say "here's my vaguely related article on my site". Now, you can imagine a meta-service that exposes websites by new users, ensures consistent usernames across all comment section on user websites, and surfaces interesting discussions in comment sections. And that's what social media is. I think tumblr matches that description most precisely