Facebook’s purchase of Instagram was brilliant, because photosharing is one of the core functions that made Facebook what it is. But Facebook was born before the smartphone explosion. Today most people take and share photos with their phones. And smartphone sales are on fire. Research firm Gartner says people bought 472 million smartphones in 2011 and fourth quarter worldwide sales were up almost 50%. Buying Instagram locks Facebook’s position as the number one photo-sharing site in the post-PC era.

Another way to view Instagram is as the hottest user-generated content (UGC) app since YouTube. Clearly people love to create and share content online. Facebook needs to make sure we keep doing that on Facebook. But creating is only one of five content related activities people do with on the social Web, which are creating, consuming, sharing, commenting, and curating.

And about that last one… Pinterest is the new curation powerhouse. Pinterest users are performing another activity core to Facebook, sharing stuff they like with friends. But Pintrest curating goes far beyond Facebook-style sharing.

When you share status updates, photos or links (hopefully you’ll share this post), it’s a one-time activity. Facebook has centered itself on this “timeline” metaphor. The things you share disappear down the page as you do more sharing over time. Pinterest has created a whole new category of Web content curation by re-imagining how to build digital collections.

Pinterest has tapped into how much people love collecting. And once most people have a collection, they want to show it off. Remember “Show and Tell” day in kindergarten? And how great it was to have a record, stamp, beanie baby, or Barbie doll collection? (Not me on the last two, just so you know.)

Pinterest grew to 10 million monthly unique visitors faster than any other website in history.  With over 20 million users today, Pinterest is now reported to have blown past everyone to become the #3 social network in the world, behind Twitter and Facebook, according to marketing firm Experian.

Facebook can ill-afford to let Pinterest stand alone, or even worse, end up in the hands of Google. Facebook’s success is based on more than just the number of users they have but also the level of engagement of those users. The amount of time people spend and the type of activities people perform on Facebook matters. Because the more data Facebook has about you, the more ads, services, and products they can market to you.

Sarah once eloquently argued that Facebook has become the Internet’s “ultimate walled garden” giving users the control to decide what they want and don’t want in their “personal gardens.” She’s right. Facebook is rapidly becoming the way people experience the Web.

But key to this “walled garden” strategy is dominating the way people create, consume, share, comment on, and curate content. Pinterest is the new category king of curation, and Facebook needs to own it. Because controlling content and all of the activities around it is key to driving user engagement and monetizing user data, two things Facebook must do to keep winning.