There's a lot regarding Pinterest's growth on Quora. For starters, see

"Which viral concepts is Pinterest getting right that may be enabling their staggering growth?" http://www.quora.com/Pinterest/Which-viral-concepts-is-Pinte...

and "Why did it take so long for Pinterest to go viral?" http://www.quora.com/Pinterest/Why-did-it-take-so-long-for-P...

Hmm, the #2 Facebook Auto-Follow hack is interesting.

I wonder if any users had a friend follow them, then mention it to them in person and have the other person be like "Oh...I never followed you."

Yes, this definitely happened. Several of us were looking at Pinterest's signup flow and how their site enabled viral features. I was the first to sign up and then my co-founder signed up later in the day, _specifically_ choosing not to follow anyone during the on boarding process. I received an e-mail from Pinterest that he was following me, then he received an e-mail that I was following him. Neither was the case.

Isn't this unethical?

I'm skeptical that the bookmarklet was a major driver in Pinterest's growth. My wife was an early and avid Pinterest user and fairly tech savvy (though not a hacker). She had no clue about the bookmarklet–or what one is–until I showed her recently.

Thats anecdotal but I suspect accurate(unless you have data to prove otherwise).

My wife was also an early adopter and an avid user. She uses the bookmarklet constantly. Be careful of trying to extrapolate from a single data point.