For me they are tools for work and they are the web apps that I use the most every day (I work remotely). Trello is a task management system where you organize tasks as drag-n-drop "cards" and Slack/Gitter are chat apps (Slack is for company chat and Gitter is for my open source project community).
I do use Youtube/Facebook/Twitter etc as well, but I use them primarily for content consumption for entertainment, so my usage patterns of those services wouldn't be affected much if they were written without using javascript at all. In contrast, drag-n-drop and the ability for real-time collaboration are the killer feature of Trello (for me), and obviously there's no way web chat would ever work smoothly without js.
I do use Youtube/Facebook/Twitter etc as well, but I use them primarily for content consumption for entertainment, so my usage patterns of those services wouldn't be affected much if they were written without using javascript at all. In contrast, drag-n-drop and the ability for real-time collaboration are the killer feature of Trello (for me), and obviously there's no way web chat would ever work smoothly without js.