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Capitol Hill hones in on Facebook today, but users around the world were abuzz about it yesterday.

For nearly six hours on Monday, Facebook stopped working after faulty configuration changes affected its internal tools and systems.

WhatsApp and Instagram - which are owned by the company - were also down.

Web infrastructure firm Cloudflare said it was as if "someone had pulled the cables from [Facebook's] data centres all at once and disconnected them from the internet".

The outage finally ended around 2200 GMT after Facebook sent a team to manually reset its servers.

In an apology last night, Facebook founder Mark Zuckerberg - who lost an estimated $6bn as shares plummeted amid the outage - wrote: "Sorry for the disruption today -- I know how much you rely on our services to stay connected with the people you care about."

We should note that the outage is not related to the leaks we have seen in the Wall Street Journal.

Facebook has also said no user data appears to have been compromised.

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Over the past few weeks, Facebook has faced a series of accusations about its internal workings.

Much of the information comes from Facebook's own internal documents, based on leaks by former employee Frances Haugen to the Wall Street Journal.

Here are five things we've learned:

• Celebrities, politicians and high-profile Facebook users had different rules governing what content they could post.
• Facebook employees regularly flagged information about drug cartels and human traffickers on the platform but the company's response was "weak".
• The company is facing a lawsuit from its own shareholders over how it resolved the Cambridge Analytica data scandal.
• Facebook may have started an initiative to "show people positive stories about the social network".
• The company conducted detailed research into how Instagram was affecting teenagers but did not share its findings when they suggested that the platform was a "toxic" place for many youngsters.

Facebook has defended itself against each accusation.

Read more about the new revelations.

David Molloy
Technology reporter

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It’s not been a good few weeks for Facebook.

There have been rumblings for a long time about a need to regulate Facebook, and its boss Mark Zuckerberg has been hauled in front of senators before.

But this hearing comes at a bad time for the tech giant.

The Wall Street Journal’s leaked documents have generated nine major scoops, external for the paper in three weeks - all potentially damaging to Facebook. The company went on a PR offensive, saying that the reporting was misleading.

But now there’s a face to the leaks. Frances Haugen has put herself into the spotlight as a real person, concerned by what she saw behind the scenes.

For those who want to bring in tough regulation, a former employee with inside knowledge calling for it herself might be a valuable source of ammunition.

And, in an unexpected twist, Facebook’s six-hour collapse on Monday means there is more attention on the tech giant - and its dominant position in global communication - than normal.

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Today’s appearance by Frances Haugen is part of a series of appearances the former Facebook product manager is making, aimed at bringing more government oversight to the company.

She has already warned that the social media giant’s driving goal is to increase engagement - even by harmful means.

“Facebook, over and over again, has shown it chooses profit over safety,” she said in an interview with 60 Minutes on Sunday.

Haugen is expected to emphasise Facebook’s push to attract increasingly younger users - a push which she says can come at the cost of their mental health.

In response, lawmakers will likely ask Haugen about the extent to which Facebook executives are aware of the detrimental effects their products have on young users.

The whistleblower who accused Facebook of prioritising profits over safety in a series of bombshell leaks is set to have her day on Capitol Hill.

Frances Haugen will testify before the US Senate today, and she's expected to call for more regulation on the tech giant - especially with regards to how younger users use its services.

We'll be bringing you live updates from the hearing, plus analysis from our correspondents. Stay tuned.