Is there concrete evidence of that or is that just the only thing that you think can explain it?
I can't dismiss that possibility, but I tend to think it's because, even though I don't see the appeal, with regular people, Pinterest is really popular and liked. In other words, the ranking seems wrong to me, but I think it might be because I am far from being a typical user.
A similar phenomenon happens with song lyrics on (regular) Google web search. If I search for "sting englishman in new york lyrics", what I want is the official lyrics from Sting's official web site (https://www.sting.com/discography/lyrics/128). Instead, I get pages of popular sites like genius.com, azlyrics.com, lyrics.com, metrolyrics.com, etc. I try to avoid these sites because their lyrics are often inaccurate and wrong (which defeats the purpose of looking up a line I'm not sure if I heard right), so the artist's official site or a fan site is vastly preferable. (I also want to support the artist. Maybe while I'm there I'll check for tour dates or merchandise.)
But I've mentioned this preference to some people before, and they surprised me by saying they like and prefer the big, well-known lyrics sites. To me, these are lowest common denominator junk bordering on spam, but to them it's what they are looking for. Point being, apparently most users aren't as particular about it as me and just want to go to familiar sites. Google may be ranking popular sites higher because that's what people actually want.
