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A new study reminds social media users to distinguish the difference between a Facebook friend and a real friend.

In fact, the report says that almost all Facebook friends are entirely fake.

The research, conducted by Robin Dunbar, a professor of evolutionary psychology at Oxford University, and published by the Royal Society, compares real-life friendship to virtual ones. According to Dunbar, for every 150 Facebook friends a user has, only four are dependable and 13 would express sympathy during an "emotional crisis." In total, only about 15 Facebook friends could be counted as real friends.

“There is a cognitive constraint on the size of social networks that even the communication advantages of online media are unable to overcome,” the report says. “In practical terms, it may reflect the fact that real (as opposed to casual) relationships require at least occasional face-to-face interaction to maintain them.”

Dunbar said that instead of growing one's social circle, Facebook and other social media platforms help slow the pace at which friendships fade. Generally, friendships that are limited in face-to-face interaction are also limited in potential to prosper.

“Friendships, in particular, have a natural decay rate in the absence of contact, and social media may well function to slow down the rate of decay,” Dunbar wrote. “However, that alone may not be sufficient to prevent friendships eventually dying naturally if they are not occasionally reinforced by face-to-face interaction.”

The study also found that younger people are more likely to have more Facebook friends, but older social media users tend to have more friends in real life. One reason for this is because social media encourages "promiscuous 'friending' of individuals who often have very tenuous links."

Plus, people who do and don't use Facebook have, on average, the same amount of friends.