Co-founder Evan Sharp said most of the users "are not just visiting Pinterest once a month, but they're actually discovering something so interesting that they're saving it or are clicking through to the website."

"It's 100 million active users, more than 70% of whom are actively engaged," he said.

The disclosure from Pinterest is the first time since its launch in March 2010 that it has discussed the number of users.

Sharp said that the majority of Pinterest users are women but noted the gender gap "is closing a little bit every month."

Based on the latest figures, Pinterest remains well behind Facebook-owned app Instagram, also a photo-sharing service, which claims some 300 million users.

But Sharp said the two platforms serve different needs and different audiences.

"Instagram is more of a social service at the end of the day," he said.

"It's more similar to a social network," while Pinterest "is much more like a search engine," he said.

"On Pinterest, you search for something, you find a great outfit, you can buy that outfit. (The two platforms) serve very different purposes."

Pinterest has taken pains to avoid being labeled a social network. Earlier this year, co-founder and chief executive Ben Silbermann described Pinterest as a "catalog of ideas" to help people discover and try new things in their lives.