For their study, the team analyzed six billion calls made by 35 million people and categorized relationships based on how frequently the subjects called each other. This was back in 2007, just a short while after Facebook opened to the public and Apple released the first iPhone, so phone calls were the most logical choice. They found that their subjects had an average of 4 people in their closest circle, 11 in the next, 30 in their third layer and 129 in their outermost circle of friends. Those numbers aren't too far off from Dunbar's estimates, and the team says they're within the layers' natural range of variation. So, if you just can't feel as close to a new friend as BFFs 1 to 5, blame your brain.