It's well-documented that humans function well with 150 friends. (The official Dunbar number is 147.8.) The "Dunbar number" comes from evolutionary anthropologist Robin Dunbar who is in the middle of a new study which is starting to prove that the size of our prefrontal cortex is directly correlated with the number of friends we have. It backs up the idea that we use our brains largely to imagine what others are thinking and how a rich social network rides along with the ability to reason about the intentions of others. Perhaps human beings evolved big brains not to understand the world, but to understand each other.
So I wonder if our brain craves to grow? I think it does. And if the growth of our prefrontal cortex is indeed social, then Facebook plays a large part of that. Which might be why we visit the site 29 times per month, 53% of us daily, and why we spend 5 hours/mo on the site. And it's also probably why 85% of marketers using Facebook find it to be successful.
Here's the current state of Facebook from their State of Union presented late last week: