In Pinterest’s five-year history, the virtual scrapbooking site has remained mum on its user numbers, only pointing the press to stats reported by third-party firms like comScore.

Turns out it was just waiting for the perfect milestone: Today (Sept. 17), Pinterest announced that 100 million people use the service each month, with the number of users, or pinners, doubling in the last 18 months. International users, which also doubled in the last year, now make up 45% of its user base.

Compared with third-party estimates, Pinterest is growing much faster than anticipated. Earlier this year, eMarketer estimated that Pintereset had 47.1 million monthly users (a representative notes this is for the US) while comScore reported 75.8 million users.

This year is shaping up to be pivotal for Pinterest. In March, new funding pushed the company’s valuation to $11 billion, stirring speculations of an IPO. Three months later, it debuted a “buy” button on its mobile app, a move that sets it up for building a potentially huge commerce business.

Facebook and Twitter are also trying to incorporate shopping into their platforms. But whereas Facebook collects moments from the past and Twitter is tied to the here-and-now, Pinterest has always been focused on the future. The aspirational nature of the site gives Pinterest a valuable glimpse into the things people want, where they want to travel, and the events they’re planning for (weddings are an obvious example).

This all adds up to users eager to hit the buy button. According to a 2014 report from e-commerce platform Shopify, the average Pinterest user spent $59 when shopping online—more than users of Facebook, Twitter, and YouTube.

Pinterest isn’t making any money off transactions on its app, and it’s told Quartz in the past it doesn’t ever intend to do so. Its sole revenue driver remains its advertising business. Though it’s only about a year old, that’s where it needs to demonstrate strong growth, especially if it eyes the public markets. That said, a viable commerce platform should endear it to advertisers.