Executive turnover was a leading indicator of its decline; before Ms Mayer was hired it went through four chief executives in three years.
Mr Zuckerberg, who controls the majority of Facebook's voting shares, is not leaving, but many top executives are.
This year several have announced their departures, including Instagram's founders;
the boss of Oculus, a virtual-reality acquisition; a co-founder of WhatsApp; and Facebook's general counsel and its chief security officer.
"The number of senior people who have left publicly and denounced the company going out the door is unprecedented.
This is Yahoo pre-Marissa Mayer," says a senior digital-advertising executive.
In another echo, the run of negative headlines is harming employee morale.
"Horrible" is how one employee describes the atmosphere at Facebook on Blind, an app where people discuss work. That raises two risks.
Star performers may leave to work at less controversial companies,
and Facebook could end up paying dearly for mediocre employees to stay on (as its share price falls, it has to hand out more in stockbased compensation to keep people).
How might things play out? Facebook is still strong but it is precariously balanced at the top of the industry, facing several big challenges in the coming year.
Above all, it must grapple with the changing ways that people use its products, which could have a huge impact on its profits.
Adults over the age of 18 are spending 31% less time on Facebook's core social network compared with two years ago, which will translate into fewer opportunities to sell ads.
A big part of Facebook's answer to this is Instagram, which executives see as a saviour. It is quickly ramping up the number of adverts users see there.
Disagreements about how strongly to push advertising on Instagram are part of the reason that the photo app's founders unexpectedly departed in October.
Now ads are around one- fifth of all posts that users see on Instagram, which is probably double what it was a year ago.
This could irk people, who may choose to spend less time on Instagram in future, just as they have done with Facebook itself.
Already users are spending more time on products that do not offer the same opportunities for advertising.
"Stories", the concept of posting sequential photos and videos of users' days and experiences that Snap, a messaging app, pioneered and Facebook copied,
are popular both on Instagram and on Facebook.
But there is less space for advertising within people's personal "stories" than there is on the conventional landing pages (called "newsfeeds")
where people scroll through posts and ads are interspersed.
Messaging apps such as WhatsApp are also growing in popularity but currently lose money.