Facebook had 829 million users in June 2014 who logged in every single day (DAUs, daily active users) (Source: Facebook's latest quarterly). There were 1.32 billion monthly active users (MAUs) as of June 30, i.e they were active at least once in June 2014. That is approx. 19% of the world's population or 47% of the part of the population that is on the internet.
There is significant evidence that suggests that Facebook use leads to negative shifts in how people feel and how satisfied they are with their lives. Researchers suspect that Facebook catalysis social comparison and envy.
Furthermore it seems that Facebook can actively change its users emotional state by tuning the Edge Rank Algorithm. A widely publicized, massive experiment with 689,003 Facebook users provided evidence that emotional contagion takes place without direct interaction between users. Only being exposed to a Facebook friend's post in the news feed is sufficient for emotional contagion to take place. The researchers reduced the amount of emotional content in the News Feed. The result was that "[w]hen positive expressions were reduced, people produced fewer positive posts and more negative posts; when negative expressions were reduced, the opposite pattern occurred."
So not only does Facebook usage itself lead to negative impacts on how people feel and how satisfied they are with their lives, but Facebook can deliberately catalyze the negative emotional shifts towards being depressed and having doubts by tweaking the Edge Rank.
This gets really interesting in the following context. Research shows, that people with doubts about themselves and their self-worth are more prone to materialism and consumerism, i.e. are more likely to buy "stuff". But it's not only that we are more inclined towards consumerism and materialism when we are unhappy, but also the other way around. Related studies show that materialism seems to negatively impact our wellbeing and satisfaction. For example because chasing after material wealth interferes with our social life. Strong consumerism can promote unhappiness because it takes time away from the things that facilitate happiness, such as time spent with family and friends. This emphasizes the systemic nature between materialism and consumerism on the one side and unhappiness and negative emotional states on the other, which one could assume work in a self-reinforcing, cyclic interdependence. This is not some kind of leftist critique of consumerism, but scientific results published in some of the more renowned psychology journals.
In light of these interdependencies the following hypothesis could be developed. People on Facebook might be more inclined to consumerism when in a negative emotional state. Thus they are more likely to click on ads and buy through the Ads displayed on Facebook. Facebook as a public company is obligated towards its shareholders to maximize revenue and profits. The vast majority of these revenues stems from selling ads. Facebook needs to find ways to maximize the number of ads being sold to advertisers. Advertisers are willing to spend more on Facebook if the revenue or number of sales that can be attributed to Facebook ads increases. So Facebook could alter its users emotional state to make people feel depressed in order to sell more ads, increase revenue and profits.
I am not implying that somebody at Facebook actively decides to make its users unhappy. But the machine learning algorithms, that are the basis for the edge rank and the ad targeting at Facebook, could automatically optimize themselves towards negatively impacting people's emotion.
Facebook is obviously willing to compromise its users' emotional wellbeing through a massive experiment for the sake of science. With this in mind, it doesn't seem like a far-fetched conspiracy theory that Facebook's data science team might not stop their algorithms from learning that unhappy users click more. Facebook's algorithm might decrease user's happiness for the sake of profits.
It would have been quite interesting to see whether ad revenues were actually higher in the group from the Facebook experiment that was shown negative posts. That could have been a quantitative validation for the thesis that unhappy people are more inclined towards consumerism. But unfortunately that information wasn't detailed in the experiment's public paper, even though I am pretty sure that the researchers at Facebook definitely looked at how much such a massive experiment impacted their core KPIs and bottom line.