One of the most important uses of Facebook is venting. When something bad happens to someone, he uses Facebook to express his concerns. This action helps the person feel better, but it spreads negativity. See why Facebook became successful.
Some people mistakenly think that by removing negative people, their Facebook newsfeed will contain positive posts. There are usually no negative or positive people, but instead a positive person can write a negative post if he has been through a bad day.
Some posts, especially the ones that express strong views, can be very tempting. Those kind of posts usually lead to negative comments and unpleasant debates. See also why Facebook is so addictive.
Irrelevant posts are annoying. When a person sees a very irrelevant post, he will usually feel annoyed even if it isn't bad or negative.
If a person is studying hard at home then sees others having fun on the beach in a Facebook post, there is a very big chance that this person will feel really bad. While the post isn't negative, it causes a lot of negative emotions. See also Study summary: Facebook makes you unhappy and jealous
Mixed messages result in unpleasant emotions. A person could see a positive post, then a negative one, then a tempting one about politics, then an irrelevant one. The combination of so many unrelated messages makes Facebook very negative.
Each day a person checks Facebook he usually has high hopes, but when he discovers that the notifications came from unwanted people he feels bad and disappointed.
When a person gets exposed to so many negative messages, most probably his mood will change to the negative too. Because negativity is contagious, Facebook users keep infecting each other with negativity. See why Facebook users constantly update their status.
All people unconsciously compare the number of likes and comments they get to their friends'. People who care too much about those factors are very likely to experience negative emotions because of believing that they are not as popular or loved as they should be.
Because the newsfeed is so random, a person can come across someone he barely likes or a person who caused him pain. By just remembering those people, a person can feel bad right away. See why some people prefer Twitter over Facebook.
After spending some time on Facebook without coming out with any additional social benefits, some people feel guilty for wasting their time or ruining their mood for nothing.
Facebook is a place where news is posted as soon as it happens. Because the news is usually negative, those who check Facebook and come across recent news might feel bad.