Beluga                                             Facebook Messenger

Yesterday’s launch of a standalone Facebook messaging app was the culmination of five months of work, which turned a fledgling app from a team of three former Google engineers and turned it into one that could interface with the daily messaging needs of the social network’s 750 million users.

We talked with Lucy Zhang, who co-founded the startup Facebook acquired to build Messenger, about how she took their original app Beluga and turned it into one that could handle messaging volume for a much larger user base. Messenger has become iOS’ top free app overnight through word-of-mouth.

Earlier this year, Zhang and her co-founders Ben Davenport and Jon Perlow launched a group messaging client called Beluga that seamlessly interlaced chat over push notifications and SMS on Android and iOS. It had thoughtfully designed hooks into the Facebook platform that helped it grow virally on the social network — which is difficult for a mobile app to do considering that there aren’t really effective viral channels on iOS.

Facebook snapped up the team in March before they could seriously consider a Series A round and the Beluga team set out to build a standalone counterpart for the social network.

That’s unusual because Facebook typically picks up a small team of engineers or product managers, puts them through engineering bootcamp and then does some matchmaking to pair them internally with a product team that fits their interests and skills. This often means the acquired company has to abandon whatever they were working on before as an independent startup.

In Beluga’s case, the three co-founders had pretty strong feelings about what they wanted from the get go. From what it sounds like, the decision to go with a standalone Facebook Messenger app was not as top-down as many of the company’s product choices are.

“When we talked to Facebook about the acquisition, one of the things we made clear was that we thought it was very important to create a fast messaging client that was dedicated to messaging,” Zhang said. “It happened that they were very aligned with what we wanted. Mark Zuckerberg wanted to make this a reality.”

Zhang said they kept most of the client-side and interface of the app the same. But they had to re-do much of the back-end to make it interact with Facebook’s messaging architecture.

In Beluga, people create  ”Pods,” where they share messages with a group of other users. But in Facebook Messages, there is a lot of user-to-user messaging and each user has their own internal copy of the message. Zhang and the Messenger engineers also decided to rely on MQTT, an open-source messaging protocol from IBM that eases bandwidth constraints and reduces latency or delays.

Beluga was also dependent on Twilio, another startup that powers voice calling and SMS-to-web interactions for many startups. Considering Facebook’s volume, it made more sense to create their own internal solution. Plus, Facebook already has relationships with hundreds of carriers globally. ”Facebook already has SMS architecture in place and a pool of short-codes we could use,” Zhang said.

Facebook is marketing the app organically at the moment. It has a landing page it publicized overnight where users can text-message themselves the link to the app’s page in Android Market or app store. The company hasn’t cross-promoted Messenger in the main app yet.

“We want to see what early adopters are saying,” Zhang said. “Once things have stabilized, we can pull many different levers to promote it.”

As for the future, Facebook hasn’t ruled out the possibility of giving mobile developers deeper access to the private messaging APIs. It’s possible they might even be used to help drive installs for third-party apps. “We don’t have specifics around that right now,” Zhang said.

For many of Android and iOS’ top downloaded apps, a Facebook integration is a nice-to-have, but not a must-have at the moment. If Facebook could drive more usage for mobile apps with more features like single sign-on or cross-platform messaging, the company might see more pick-up among third-party developers.