With over 500 million users worldwide, Facebook has become a global phenomenon, a vast cyber neighborhood where friends meet to share photos, news and gossip. But when those relationships sour, another phenomenon often occurs -- unfriending.

In what may be the first comprehensive study of its kind, a University of Colorado Denver Business School student has revealed the top reasons for Facebook unfriending, who is unfriended and how they react to being unfriended.

"Researchers spend a lot of time examining how people form friendships online but little is known on how those relationships end," said Christopher Sibona, a PhD student in the Computer Science and Information Systems program whose research will be published January by the Hawaii International Conference on System Sciences. "Perhaps this will help us develop a theory of the entire cycle of friending and unfriending."

After surveying more than 1,500 Facebook users on Twitter, Sibona found the number-one reason for unfriending is frequent, unimportant posts.

"The 100th post about your favorite band is no longer interesting," he said.

The second reason was posting about polarizing topics like religion and politics.

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"They say not to talk about religion or politics at office parties and the same thing is true online," he said.

Inappropriate posts, such as crude or racist comments, were the third reason for being unfriended.

The study showed 57 percent of those surveyed unfriended for online reasons, while 26.9 percent did so for offline behavior.

Sibona found a sort of online hierarchy of dominant and subordinate relationships. For example, those making friend requests stood a much higher chance of being abruptly unfriended.

At the same time, those doing the unfriending seemed to hold the upper hand in the relationship.

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It's a delicate dance with its own rules or "nettiquette," far different from face-to-face interaction.

"There is a lot more nuance in the offline friendship world. You don't have to go up to someone and ask them to be your friend," Sibona said. "That's not the case online. It can be awkward."

An AOL study showed 30 percent of teenagers wanted to unfriend their own parents. Sibona found two users who actually did this. One later refriended his mom but put her on a limited profile so he could manage her online interactions.

While some respondents reported being deeply hurt at being unfriended, others were more amused than traumatized.

"There are a wide variety of reactions depending on who did the unfriending and why," he said.

Facebook , founded in 2004 by Mark Zuckerberg, is so ingrained in popular culture that in 2009 unfriend was named word of the year by the New Oxford American Dictionary, which defined it as "to remove someone as a `friend' from a social networking site such as Facebook." A movie about Zuckerberg, The Social Network, was released last week.

Given the public nature of Facebook profiles, Sibona urged users to exercise caution in their posting behaviors citing a 2010 survey showing that 54.6 percent of recruiters used the site to find or investigate job candidates.

"The same kinds of posts that could get you unfriended might also be viewed negatively by recruiters," he said.

Steven Walczak, associate professor of Information Systems at the University of Colorado Denver Business School and Sibona's advisor, said he hopes the study will spark further research.

"With businesses embracing Facebook as a marketing and customer-relationship tool, this will hopefully create new research that further examines how social networks enhance business decision making and outcomes," he said.