So the main point - allegedly - of Metafilter is the community. Do they get to Metafilter by going to Google first? Why would they? I don't Google my favourite destination sites, e.g. BBC news, I just type 'b' in the browser and I am kind of there. Same with HN, I just type 'n' and I am a down-arrow + enter away from being there.
I just don't see how a few changes to a search engine can instantly result in a 40% of traffic to a community site. I could understand if it was a 40% drop in new members but not overall traffic.
Incidentally, I have used the web since 1993 - twenty years - and I cannot ever remember going to Metafilter from a search. I have seen the site, and I have some perception it is like 'tripod' or 'geocities', as in ancient. Although anecdotal, I don't believe that they get search traffic in a big way or that they have done so for years. I have also never gone to 'Reddit' from a search result - community sites should and do rely on word of mouth introduction (or links).
The oft-criticised Alexa stats on Metafilter tell a story when compared to SO, Reddit and Quora. The bounce rate is high, the page views low and the time on site low. IMHO there are a lot of people that just do not like the look of the site It needs a redesign with the existing look kept as 'low bandwidth' option to allow the community that likes the existing look to stay with what they know.