What she is (in hindsight) expected to have done is to have insisted that Facebook's news feed algorithm be used as a content filter to suppress stories promoted by politically marginalized groups.
This is much the same way China's government uses its Great Firewall to control which information reaches the people.
During the civil rights era, the Soviets supported the work of Martin Luther King. In the modern world, Russia supports certain third party views and anti-establishment perspectives, for obvious reasons, just as the US supports pro-Ukraine groups and other fringe political groups that are in close orbit to Russia.
So we are being told that Sandberg failed to utilize the firewall capabilities of Facebook to suppress anti-establishment content. We are told this is a big deal because Russia-linked entities spent money to promote some of the ads. Keep in mind the total spend by Russian-linked entities (could be NGOs, private citizens, oligarch-funded research institutes, etc.) was a miniscule $150K in an election with total funding approaching $2B.
Yet Sandberg may be relieved of her position because she failed to anticipate how such a small spend could dramatically influence the outcome of the 2016 election.
News flash: It didn't. There are a lot of reasons to be concerned about some aspects of Facebook -- my #1 concern is the extensive use of dark patterns designed to make people over-share -- but the "not doing enough" critique is complete bunk.
It is 100% motivated by political partisans in the US who unfortunately have a great deal of influence in possible regulation of Facebook and have made it very clear that they plan to use all of their power to force Facebook to act like the well behaved Great Firewall they want.
To be very clear, politicians want Facebook to be used at their bidding as a Great Firewall and they very much want to see certain views suppressed and others amplified. They also want to (and already do) use the massive amounts of metadata held by Facebook, Google, etc., to supplement whatever data they have when investigating Americans, who may or may not have committed a crime. It is, essentially, a transparent social credit system for US officials to use at their whim.
So the only difference between Facebook as politicians want it to be used and China's Great Firewall is that Facebook exists a bit higher up the protocol stack and its censorship/suppression is much more laser-focused and harder to detect.