Pinterest has been around for five years and it’s been obvious the site has attracted a lot of attention and users. Until now, though, the company has been shy about announcing user numbers, pointing people to third-party firms like comScore rather than announcing user numbers directly. Now, Pinterest is changing its tune, according to Quartz: “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 percent of its user base.”

That’s a rapid rate of growth when you look back at earlier estimates from third-parties like eMarketer, which earlier this year estimated that Pinterest had 47.1 million monthly users in the US, while comScore reported 75.8 million users in total.

The attraction of marketers to Pinterest is simply the 100 million users — it’s what those users are there for. Pinterest represents what people are interested in in the future, and particularly things they want to purchase. Pinterest has teamed up with fashion retailer Topshop to create a color palette tool based on saved pins. As VentureBeat reports,”Pinterest could further enhance its advertising efforts with promoted pins and its ads API by using user data derived from these third-party sites.” Ads on Pinterest blend seamlessly with the content — it’s more like native advertising than the way ads appear on other social media.

Since Pinterest added “buy” buttons to its mobile app in June, it’s clearly beginning to capitalize on its potential power as a shopping platform. A report in 2014 from Shopify showed (above) just how good Pinterest is at influencing purchases — Pinterest beat out Facebook, Twitter, and YouTube when it came to user spend. Marketers should definitely be looking more closely at Pinterest for a variety of reasons.