The vast majority of Metafilter pageviews are people who lurk and just read the content. (The $5 barrier to entry and reasonably small community is a feature, not a bug)

The vast majority of these lurkers originally hit the ask.metafilter site by searching for a specific question and landing on a thread where a MeFi member has asked the same thing. Those users were the ones being targeted by ads. So yes, it's pretty easy for a google algorithm change to reduce traffic by that much.

They aren't optimizing the site for search traffic or for growth. They're optimizing it for fantastic discussion and community. I realize that's a foreign concept here on HN, where growth and monetization are king, but I believe it's why Metafilter has remained a popular and influential site for over 15 years. (That's like 100 in internet-years...)