Facebook CEO Mark Zuckerberg on Tuesday testified in front of the US Senate. When Zuckerberg was asked about Russia's 2016 U.S. Presidential election, he said: "We were too slow to spot and respond to Russian interference, and we\u2019re working hard to get better. Our sophistication in handling these threats is growing and improving quickly." He added that Facebook is significantly increasing their investment in security. "We now have about 15,000 people working on security and content review. We\u2019ll have more than 20,000 by the end of this year," he added. The Facebook CEO apologized for what happened and took responsibility for everything. He also said that there is an online propaganda "arms race" with Russia and it was important to make sure no one interferes in any more elections including in India, Facebook CEO Mark Zuckerberg said at a five-hour marathon session at the US Congress. "The most important thing I care about right now is making sure no one interferes in the various 2018 elections around the world," he testified before a 44-Senator panel. "As long as there are people sitting in Russia whose job it is to try and interfere with elections around the world, this is going to be an ongoing conflict," the 33-year-old billionaire said as he prepared to testify again, this time before a House panel. Zuckerberg has said Facebook will ensure that its platform is not misused to influence elections in India and elsewhere. Facebook on Monday said: "Our goals are to understand Facebook's impact on upcoming elections - like Brazil, India, Mexico and the US midterms - and to inform our future product and policy decisions." Facebook's stock was up about 2 percent even before Zuckerberg sat down. It moved even higher when he started addressing the questions from lawmakers and finished the day with a 4.5 percent gain. Zuckerberg accepted that the company did not do enough to prevent the platform from being used to harm others. In his opening remarks, he said: "Facebook is an idealistic and optimistic company. For most of our existence, we focused on all of the good that connecting people can do. "But it's clear now that we didn't do enough to prevent these tools from being used as harm as well. His apology came as Facebook faced a widening scandal where a British political consultancy firm Cambridge Analytica improperly gathered detailed information on 87 million of its users. "It's not enough to just connect people. We have to make sure those connections are positive. It's not enough to give people a voice. We have to make sure people aren't using it to harm people or spread disinformation," Zuckerberg told Senators.