Even a very 'light' definition would catch YouTube, I'm convinced of this. The UK's definition is—broadly—any site that a user can take an action on that would affect other users. This would definitely catch a forum like HN, any site with comments, etc. Personally, I feel that, combined with draconian identity requirements, that goes way too far, but I think I'd struggle to draw a line that better fits the alleged intent of these political moves.