This is a really interesting question. I'd never really thought about it.
Perhaps it's just Google being spiteful as it's people use it more than G+
(Not really what I think but I don't understand why.
Maybe Google throttle back PR increases for new sites. I suppose as Pinterest is relatively new, although it has many good quality links, Google may not let achieve a greater PR than 6 for the first couple of years!? This is just wild speculation by the way.
