While Facebook CEO Mark Zuckerberg seemed cheerful and even jokey when he took the stage today in front of journalists and media executives (at one point, he described the event as “by far the best thing” he’d done this week), he acknowledged that there are reasons for the news industry to be skeptical.

Facebook, after all, has been one of the main forces creating a difficult economic reality for the industry over the past decade. And there are plenty of people (including our own Josh Constine) who think it would be foolish for publishers to trust the company again.

For one thing, there’s the question of how Facebook’s algorithm prioritizes different types of content, and how changes to the algorithm can be enormously damaging to publishers.

“We can do a better job of working with partners to have more transparency and also lead time about what we see in the pipeline,” Zuckerberg said, adding, “I think stability is a big theme.” So Facebook might be trying something out as an “experiment,” but “if it kind of just causes a spike, it can be hard for your business to plan for that.”

At the same time, Zuckerberg argued that Facebook’s algorithms are “one of the least understood things about what we do.” Specifically, he noted that many people accuse the company of simply optimizing the feed to keep users on the service for as long as possible.

“That’s actually not true,” he said. “For many years now, I’ve prohibited any of our feed teams … from optimizing the systems to encourage the maximum amount of time to be spent. We actually optimize the system for facilitating as many meaningful interactions as possible.”

For example, he said that when Facebook changed the algorithm to prioritize friends and family content over other types of content (like news), it effectively eliminated 50 million hours of viral video viewing each day. After the company reported its subsequent earnings, Facebook had the biggest drop in market capitalization in U.S. history.

Zuckerberg was onstage in New York with News Corp CEO Robert Thomson to discuss the launch of Facebook News, a new tab within the larger Facebook product that’s focused entirely on news. Thomson began the conversation with a simple question: “What took you so long?”

The Facebook CEO took this in stride, responding that the question was “one of the nicest things he could have said — that actually means he thinks we did something good.”

Zuckerberg went on to suggest that the company has had a long interest in supporting journalism (“I just think that every internet platform has a responsibility to try to fund and form partnerships to help news”), but that its efforts were initially focused on the News Feed, where the “fundamental architecture” made it hard to find much room for news stories — particularly when most users are more interested in that content from friends and family.

So Facebook News could serve as a more natural home for this news (to be clear, the company says news content will continue to appear in the main feed as well). Zuckerberg also said that since past experiments have created such “thrash in the ecosystem,” Facebook wanted to make sure it got this right before launching it.

In particular, he said the company needed to show that tabs within Facebook, like Facebook Marketplace and Facebook Watch, could attract a meaningful audience. Zuckerberg acknowledged that the majority of Facebook users aren’t interested in these other tabs, but when you’ve got such an enormous user base, even a small percentage can be meaningful.

“I think we can probably get to maybe 20 or 30 million people [visiting Facebook News] over a few years,” he said. “That by itself would be very meaningful.”

Facebook is also paying some of the publishers who are participating in Facebook News. Zuckerberg described this as “the first time we’re forming long-term, stable relationships and partnerships with a lot of publishers.”

Several journalists asked for more details about how Facebook decided which publishers to pay, and how much to pay them. Zuckerberg said it’s based on a number of factors, like ensuring a wide range of content in Facebook News, including from publishers who hadn’t been publishing much on the site previously. The company also had to compensate publishers who are taking some of their content out from behind their paywalls.

“This is not an exact formula — maybe we’ll get to that over time — but it’s all within a band,” he said.

Zuckerberg was also asked about how Facebook will deal with accuracy and quality, particularly given the recent controversy over its unwillingness to fact check political ads.

He sidestepped the political ads question, arguing that it’s unrelated to the day’s topics, then said, “This is a different kind of thing.” In other words, he argued that the company has much more leeway here to determine what is and isn’t included — both by requiring any participating publishers to abide by Facebook’s publisher guidelines, and by hiring a team of journalists to curate the headlines that show up in the Top Stories section.

“People have a different expectation in a space dedicated to high-quality news than they do in a space where the goal is to make sure everyone can have a voice and can share their opinion,” he said.

As for whether Facebook News will include negative stories about Facebook, Zuckerberg seemed delighted to learn that Bloomberg (mostly) doesn’t cover Bloomberg.

“I didn’t know that was a thing a person could do,” he joked. More seriously, he said, “For better or worse, we’re a prominent part of a lot of the news cycles. I don’t think it would be reasonable to try to have a news tab that didn’t cover the stuff that Facebook is doing. In order to make this a trusted source over time, they have to be covered objectively.”