If you're happy and you know it post on Facebook...
In many ways, the internet has brought us all closer together. The use of WebLogs and various social media sites make it possible to instantly interact with people around the world. Despite this and other benefits, there can be some drawbacks. For example, a study recently published in the Personality and Social Psychology Bulletin suggests that social networking sites such as Facebook.com can have a negative impact on an individuals self esteem and overall feeling of well being.
Through a series of studies, Jordan and coworkers found that when someone is feeling down, those feelings can be enhanced when an individual perceives others around them to be happier and problem free. The researchers suggest this perception can be prevalent in social websites where users have complete control over the public image they project. Although the various investigations in this work had a fairly limited sample size (between ~60-300 participants for the various portions)it is certainly worth a read and some thought.
If you're interested in learning more about psychology and ways to combat problems with negative thinking, there are some excellent articles and resources at my wife's psychology blog.