The details of this study can be found in the Circle of Friends Report, commissioned by The National Lottery. I wouldn't begin adding or subtracting friends just yet as this might just be another case in which psychologists confuse correlation with causation; evidence suggests that the common cause here might be an extroverted personality-type. There might also be a self-reinforcing cyclical relationship of some sort: happy people are more successful, socially, and social success makes people happier.
Excelsior!
Social Networks