This may be addressed(corporate firewall blocking me) but how much is that driven by Google Image Spam though? I honestly can't think of the last time someone has even mentioned Pinterest to me, and most of my friends are female.
I've seen lots of highly active accounts posting hundreds of images. That's the spam I was referring to. Pinterest seems uninterested in policing/removing them, which makes sense because they draw traffic.
I suspect those are counted as MAUs (and I can't see any reason Pinterest wouldn't count them, even though a human can tell they're just bot accounts).
Unless Pinterest has written transparently about their massive bot problem, I'm reluctant to believe any self-serving metric they publish.
Comscore reports ~111 million monthly uniques for US only, so 175 million MAU seems plausible, though I'm sure you are right that it includes a few million bots.
