Facebook CEO Mark Zuckerberg took to the stage tonight at the TechCrunch Disrupt conference in San Francisco, fielding questions from TechCrunch founder-turned-investor Michael Arrington, in what turned out to be a sparky interview.

What would he have done differently with Facebook’s IPO? “The performance with the stock has obviously been disappointing,” he said, while maintaining Facebook is determined to “do the things that are going to build value over the long term”. And that is? “Really how well we do with mobile”.

Zuckerberg said that it’s “easy for people to underestimate how fundamentally good mobile is for us right now… on a bunch of different levels”.

First because there are more users, and second because people who use Facebook on mobile have more engagement – they’re spending more time using it. And third, that Facebook is going to make more money from those users.

Is it really a strength and opportunity, or a weakness? “Mobile is… there are going to be more users, each user is going to spend more time, and per amount of time that they spend, we’re going to be making more money,” said Zuckerberg.

“Since we did the new version fo the iOS app, which makes what we’re doing competent, we’re already seeing double the amount of feed stories consumed per user on a daily basis.”

He said that “mobile is a lot closer to TV than desktop” when it comes to monetisation. So Facebook is making billions of dollars from a sidebar of ads on its website, but on mobile that format has to change – “they have to be fundamentally integrated into the product”, and the ones tried so far are already performing better than those on the website.

“We’ve had a bunch of mis-steps on this,” he admitted. “When I’m introspective about the last few years, I think that the biggest mistake we made in the company was betting too much on HTML5 versus native. It just wasn’t there.”

Zuckerberg stressed that Facebook’s mobile website is still a big thing for the company, but explained that the company was never able to get the quality it wanted from HTML5 in its native mobile apps. “One of the biggest mistakes if not the biggest strategic mistake that we’ve made.” But the new Facebook native iOS app – with Android to come – is sorting that out.

Facebook still has more usage on its mobile website than on its native iOS and Android apps put together. But Zuckerberg said “you can do so much better on iOS and Android by doing native work… We’re very self-critical, and it was not where we wanted it to be before.”

There is more to come. The current iOS app essentially has the same features as the previous HTML5-based version, just faster, but Zuckerberg promised “a lot of cool stuff” in the coming months, based on work done by other teams within Facebook on apps.

“Now we are a mobile company,” he said. “All the code that’s being written is mobile.”

Facebook has now closed its acquisition of photo-sharing startup Instagram. What are its plans for the app?  ”They just crossed 100m registered users,” noted Zuckerberg – a new milestone, as far as we’re aware.

“We think Instagram is amazing, and we wanna help it grow to hundreds of millions of users… We’re going to try to do the things we would’ve done if they were an Open Graph partner, but just prioritise them.”

What about that long-rumoured Facebook Phone? Is the company building its own smartphone? Zuckerberg said no. “It’s always been the wrong strategy for us. We’re building this network, we have 950m users… Let’s say we build a phone, theoretically – we’re not – we get 10m people to use it. It doesn’t move the needle for us!.. We wanna build a system that is as deeply as possible integrated into every major device.”

But he came back to his core theory that mobile users of Facebook are more engaged, and that this is having an impact on what Facebook’s developer partners are doing. “A lot of the energy of the ecosystem is not going towards building desktop stuff any more, it’s going towards building mobile stuff… That’s the future.”

But back to that phone? “The phone just doesn’t make any sense!”

Zuckerberg was asked about developers, including Zynga, which he admitted has had a rough time recently. However, Zuckerberg was positive: “We have more people playing games on the Facebook platform now than in the past, we’re up to 230m people playing monthly,” he said.

Zuckerberg also pulled out Spotify – “Spotify is killing it” – Airbnb, Nike and Runkeeper as companies doing innovative work with Facebook’s platform outside the social network itself.

The last question: Is Zuckerberg still having fun? “For me, it’s not really about fun. It’s about mission.”

Picture source: TechCrunch Disrupt livestream