Recent research by social psychologists suggests that a large and complex virtual social network (such as can exist on Facebook), combined with real face to face social interaction, creates and maintains a healthier brain.
Bickart et al. have discovered that the size of your social brain, called the amygdala, is positively associated the size of both virtual and physical social network size. There is also suggestion from the research that maintaining these relationships could enhance the size of this area of your brain.
As the Amygdala communicates with other parts of the brain there are connections being formed and over time repeated communication strengthens those connection. Particularly in older populations, this strengthening could have a very therapeutic effect. It is like a work out for your social brain. It's maintaining what you have and helping you hold on to it.
Again, these correlations between the size of your social brain (amygdala) are based on the size and complexity of your social networks (virtual and Physical. Complexity means how many different groups you maintain relationships in (ex. Church, school, work, hobbies, causes, ect.). Size is how many people you actually communicate with on a daily or weekly basis. They are positive correlated which means lots of communication equals big brains.
There is a lot of work yet to be done in this area of social research but I think these are some very interesting findings.