Paul Monheimer points out that this month's National Geographic features an article on the teenage brain, which is available online here.
Dan Griffiths points out the recent study, "Online social network size is reflected in human brain structure" that yes, shows a positive correlation between number of Facebook friends and gray matter density in certain areas of the brain involved in social perception and associative memory.
Finally, check out the recent report from British researchers on how verbal & non-verbal IQ can fluctuate throughout the teenage years. Here's a summary of their work:
"The mental ability of teenagers can improve or decline on a far greater scale than previously thought."