One difference is that Twitter actively promotes trending topics in the sidebar, so it's as if someone noticed that multiple people in the town square were shou

One difference is that Twitter actively promotes trending topics in the sidebar, so it's as if someone noticed that multiple people in the town square were shouting about the same thing and circulated it throughout the town, not just the square, just because of that. Twitter has always exercised some editorial control over that trending topics sidebar, but it's never been clear to me how much.