Now that the massively popular photo-sharing app he helped build has been purchased by Facebook, Instagram cofounder Mike Krieger is taking some time out to share tips with fellow mobile app developers.

In an on-stage interview Thursday at the GigaOM Mobilize conference in San Francisco, Krieger said mobile app companies trying to grow need to consider what to specialize in, and what can be outsourced to others. 

For Instagram, which rolled out in 2010 and now has 100 million users who have posted 5 billion photos, it became apparent early on that it was necessary to focus on making the app start and show content quickly, since people tend to use it briefly at bus stops or on public transportation, he said.

“That minute has to be magical; it can’t be loading,” he said.

In addition to speed, knowing where people are going to be using your app is important, he said, as the apps often tend to be “filling in these gaps in people’s lives,” he said. “You want to make the day a little bit better.”

Asked by moderator Om Malik (founder of the GigaOM blog network) what he thinks developers are doing wrong with mobile apps, he said they sometimes get too fancy with the first version, cautioning developers against over-thinking things.

Krieger declined to comment on whether developers should make their app for Google’s Android operating system or for Apple’s iOS first, but said they had to pick a side. Offering an app for both early on slows a company down, he said. Instagram was initially only available for the iPhone, but is now available for both platforms.

One funny note: When asked why Instagram doesn’t seem to crash much, Krieger cited the service’s simplicity and reliance on trusted technologies, but he also said that operating as a “mobile first” company can hide some instability from users. If there’s a two-minute blip in the service, he said, people just think it’s an issue with their wireless service.