"You can use secret boards to keep track of holiday gifts, plan a special event, or work on a project you aren't yet ready to share with the rest of the world."
Pinterest was gradually rolling out a test feature that lets users create as many as three secret boards, according to Milam.
Anything "pinned" to secret boards doesn't show up elsewhere at Pinterest and only people invited by creators can add to the typically theme-based online collages.
Japanese online giant Rakuten early this year announced that it was leading a $100 million investment in Silicon Valley-based Pinterest that would allow the startup to improve services and expand globally.
Rakuten said it was joining with existing investors Andreessen Horowitz, Bessemer Venture Partners, and FirstMark Capital, and "a number of angel investors."
The hot young website, particularly among women, gives people virtual bulletin boards that they decorate with pictures showcasing interests in anything from food to sports, fashion or travel.
It does not release use figures but was estimated to have 20 million or so unique visitors and some see potential for the site to cash in on e-commerce.