The difference for me has always beem that Twitter is organized by person rather than by topic. In some ways that's good if I'm following someone because I care about them personally, but usually I follow people because I'm interested in a particular aspect of their work like art or announcements. In those cases, I don't want to hear about their political opinions or what their child did that week. I know you can follow topics as well, but that often seems overwhelming and imprecise. Social media as a platform is just incredibly confusing in the way it blurs the line between performer and audience.