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Facebook offers peek into App Center recommendation engine

When Facebook rolled out its new App Center this summer, it included a built-in recommendation engine to help users uncover the perfect apps.

But how does it work? The social network's engineering team has finally provided a glimpse of the engine's inner workings.

On average, 220 million people visit the App Center each month, according to the Facebook Engineering team, and given the multitude of Facebook-integrated apps, making sure people receive the right recommendations can be a challenge.

"We needed to build a system that could handle large-scale data and traffic, respond quickly, and incorporate user feedback in realtime," the engineering team said in a blog post. "The goal is for curation of the App Center to be driven by quality and personalization, instead of editorialization."

The recommendation engine taps into users' preferences in order to serve them apps that are timely, socially relevant and unique. "This allows a more diverse set of apps to become discoverable, particularly those in harder to find or up-and-coming categories," Facebook said.

Similar to the architecture of a search engine, the system follows an "aggregator-leaf" format, the Facebook Engineering blog said. The aggregator acts as a central controller, collecting recommendation requests and distributing them to various subsets. In the end, the system merges and returns what it believes are the best results for the consumer. User feedback is then factored into the results over time.

"Growth in the App Center is tied to quality," the blog said, "and we determine that quality based on user ratings and positive/negative user signals for an app over time."

Users are asked to rate apps shortly after Facebook learns that they used it. "Then, when we compute the average rating for an app, we include a confidence adjustment based on the number of ratings the app has received," Facebook said.

Daily active use is usually a good indicator of popularity, Facebook said, because monthly data might include a one-time spike. As a result, app quality is largely determined by average rating and average daily active users.

The App Center launched in June, and was rolled out to all users by 1 August, delivering hundreds of apps, including Nike+ GPS, Ubisoft, Stitcher Radio, Draw Something, and Pinterest, for mobile and Web users.