Google: a lot, most of their services has one. However, I believe they use an OAuth (-ish) service rather than session IDs to manage authentication, so cookies

Google: a lot, most of their services has one. However, I believe they use an OAuth (-ish) service rather than session IDs to manage authentication, so cookies aren't really an issue. YouTube: no major subdomains as far as I know. Facebook: aside from developer resources (which require you to log on with Facebook), none really. Baidu: not familiar with it, so no idea. Wikipedia: one for every language. Furthermore, the Wikipedia of each language seems to be completely separate, both in content and accounts. Reddit: a lot. Aside from the obvious api.reddit.com, users may use <whatever>.reddit.com and the subreddit's CSS may use this info to change the looks of the subreddit. Instagram: no idea, but I believe the main interface is simply on instagram.com. Netflix: just netflix.com. Twitch: as far as I know just the main domain, no subdomains. Spotify: a few services, like open.spotify.com and play.spotify.com, but they all require you log on separately. It's pretty mixed, with some websites having a lot of subdomains, but not all of them requiring or using shared cookies.