Online Gaming: This is its natural habitat. When a player makes an excellent move, teammates will often flood the chat with “666” to show their appreciation.
Live Streaming (直播 - zhíbō): When a live streamer (主播 - zhǔbō) does something impressive—sings a high note perfectly, wins a difficult game, or tells a great joke—the comment section (弹幕 - dànmù) will be filled with a stream of “666” from viewers.
Social Media and Messaging: It's commonly used in comments on Weibo, Douyin (TikTok), and WeChat to react to anything impressive. This could be a beautiful piece of art, a funny video, a clever retort, or someone sharing a personal achievement.
Spoken Language: While it started online, “666” has crossed over into spoken Mandarin, especially among younger people. You might hear someone exclaim “liù liù liù” in real life to informally praise a friend.
Connotation & Formality: The term is overwhelmingly positive but highly informal. It's perfect for friends, peers, and online communities, but would be out of place in a formal business meeting, an academic paper, or when speaking to an elder who isn't familiar with internet culture.