Facebook and Twitter said they failed to stop the spread of fake news and propaganda on their platforms. The companies' bosses testified in front of the US Senate's Intelligence Committee and said they were improving their systems. But many lawmakers want to beef up regulations on the sector. As Mobin Nasir reports, that is spooking investors.

For more on the future of social media, we spoke to Rebecca Lieb, a media analyst and co-founder of research firm Kaleido Insights.

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