Facebook CEO Mark Zuckerberg's recent manifesto, which described a vision for a more privacy-focused Facebook, raised questions around how the social network could change.

Of all people, recently departed Instagram founders Kevin Systrom and Mike Krieger have more insight than most on how Facebook's most popular services could evolve in the future. Facebook (FB) - Get Report shares were up 1.46% on Monday.

Speaking at the SXSW conference in Austin, Systrom and Krieger -- who left Facebook last fall -- reflected on Facebook's $1 billion acquisition of Instagram in 2012 and made the case that it was overall better for the Instagram user base, which has exploded in size to over a billion users. In addition to the app's popularity, Instagram is also increasingly the crown jewel of Facebook's advertising business.

The Instagram founders were asked about the notion of integrating Facebook's messaging services -- Instagram, Facebook Messenger and WhatsApp -- rather than having them exist as separate services with separate sign-ons. In his essay last week, Zuckerberg wrote that boosting the interoperability of those services would "improve convenience in many experiences where people use Facebook or Instagram as their social network and WhatsApp as their preferred messaging service."

Systrom seemed to agree with the idea that more closely integrating the messaging features could make them more useful, but suggested that it's not a sure thing that people would actually use the integrated messaging.

"Most of the integration I believe they're talking about is on the messaging level," Systrom said in the interview. "The thesis is the more people that are available to talk with, the more useful the platform becomes. And I buy that thesis. The question is whether people sign on to separate platforms, whether it's Instagram, WhatsApp or Facebook, under different identities, whether or not they will in fact want to talk to people on different platforms -- I think we're going to have to see how that plays out."

Facebook does update investors on the volume of users on Facebook, Instagram and WhatsApp, but doesn't parse out how many of those identities are unique or tied to a single person. And in the interview, the Instagram founders noted that it was clear early on that many people on social media platforms maintain several accounts.

Apart from the argument for privacy and encryption that Zuckberberg laid out last week, Krieger also pointed out a business case for integrating Facebook's messaging services -- for example, in countries such as Brazil where WhatsApp is heavily used, businesses that promote products on Instagram but maintain a WhatsApp account may benefit from integration.

"There are a bunch of opportunities to make that easier," Krieger added. "If they're able to prove out some of these ideas without doing the architectural work that would be better."

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