They're basically using Facebook for emotional support.

People who aren’t emotionally stable post more frequently in order to regulate their emotions and receive social support.

Low emotionally stable individuals experience emotions more intensely and have difficulty regulating their emotions on their own. Consequently, we suggest that they use the microblogging feature on online social networks (e.g., Tweets or Facebook status updates) to help regulate their emotions. Accordingly, we find thatless emotionally stable individuals microblog more frequently and share their emotions more when doing so, a tendency that is not observed offline. Further, such sharing, paired with the potential to receive social support, helps boost their well-being.

And those people posting all those status updates to regulate their emotions may not be aware of the main reason for un-friending:

Posting too many boring updates.