According to its Developers Blog, Facebook has been driving some serious traffic to integrated third party sites. Last month, we reported that out of the social giant’s 425 million monthly mobile users, as many as 60 million are driven to apps created by 3rd party developers, adding up to a whopping 14% of Facebook’s total monthly mobile traffic.

Today, the developer spotlight is on The Guardian‘s app, which has grown tremendously to 3.9 million active users, over half of which are under the age of 25 (a traditionally hard to reach audience for the publication). Note that we reported a few months back how The Guardian had reached 4 million app instillations in two months. Today’s announcement shows that installs and active users are two very different things.

The Guardian’s app integrates seamlessly with the social graph, which is why Facebook says it got it right. More specifically, The Guardian embraced lightweight actions like ‘agree’ and ‘disagree’ for opinion articles, and beefed up its article pages with Facebook’s Facepile and Activity Feed plugins. All of these integrations keep readers coming back to Facebook, but also appear to be helping publications become more social than ever before.

attachment 520x218 The Guardian’s Facebook app hits 3.9 million active usersOther successful integrations that Facebook is touting include Yahoo! News, which has seen a 600% increase in traffic coming from Facebook and Washington Post, which has more than 3.5 million monthly active users for its Social Reader app.

All in all, tons of apps and publications are seeing massive growth since they integrated with the Facebook Platform. For developers, this means it’s time to jump on the bandwagon while the getting’s good, so long as they’re careful not to spam the heck out of users.

The major risk here is having to find the balance between spam and growth, as the Facebook Platform becomes more crowded over time. Whether this growth is sustainable or not is still unclear, but at the moment, the results have clearly been phenomenal for Facebook and its partners — all while strengthening Facebook’s stronghold as a social platform.