The general rule for "meta" is: "meta-X" → "X about X." Examples: "meta-data" → "data about data", "meta-language" → "language about language", "meta-vanity" → "vanity about vanity" (a concept explored by the late DFW though he termed it "second-order vanity"), etc.

Talking about Hacker News... about the website, about social media etc.

'Meta' because instead of just hacker news, we have news about hacker news. (from metadata)