Facebook Inc. chief executive Mark Zuckerberg pledged to stop the misuse of user data on its site to manipulate voters in India, Brazil and the US.

In a Facebook post on Wednesday, Zuckerberg apologized for mistakes his company made in how it handled data belonging to 50 million of its users, and promised to take tougher steps to restrict developers’ access to such information. The company, Zuckerberg said, “made mistakes, there’s more to do, and we need to step up and do it".

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“This was a major breach of trust. I’m really sorry this happened. We have a basic responsibility to protect people’s data," Zuckerberg said.

Facebook shares fell 1.8% in early trading on Thursday as the apology failed to quell Wall Street nerves.

What Zuckerberg said

On regulation: “I actually am not sure we shouldn’t be regulated. There are things like ads transparency regulation that I would love to see."

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On how many apps Facebook will investigate: “It will be in the thousands."

On plans to avoid foreign interference in the US midterm elections: I feel a lot better about the systems now. At the same time, I think Russia and other governments are going to get more sophisticated in what they do, too. So we need to make sure that we up our game. This is a massive focus for us to make sure we’re dialled in for not only the 2018 elections in the US, but the Indian elections, the Brazilian elections, and a number of other elections that are going on this year that are really important.

On Facebook’s basic economic model: I don’t think the ad model is going to go away, because I think fundamentally, it’s important to have a service like this that everyone in the world can use, and the only way to do that is to have it be very cheap or free.

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On the #DeleteFacebook campaign: I don’t think we’ve seen a meaningful number of people act on that, but, you know, it’s not good. I think it’s a clear signal that this is a major trust issue for people, and I understand that. And whether people delete their app over it or just don’t feel good about using Facebook, that’s a big issue that I think we have a responsibility to rectify.

What his critics said:

This isn’t going to cut it. Mark Zuckerberg needs to testify before Congress- David Cicilline, Democratic US representative from Rhode Island
I demand clarification on whether German users and accounts are affected and what Facebook plans to do to prevent this from happening again- Katarina Barley, Germany’s justice minister
It shouldn’t be for a company to decide what is the appropriate balance between privacy and innovation and use of data- Matt Hancock, Britain’s minister for culture and digital