This NYT piece is rather interesting & slightly depressing, if mostly heteronormative.
That thought struck Lisa Degliantoni, an educational fund-raising executive in Chicago, a few months ago when she was planning her 39th birthday party. After a move from New York to Evanston, Ill., she realized that she had 857 Facebook friends and 509 Twitter followers, but still did not know if she could fill her party’s invitation list. “I did an inventory of the phases of my life where I’ve managed to make the most friends, and it was definitely high school and my first job,” she said.
It was found that:
…in studies of peer groups, Laura L. Carstensen, a psychology professor who is the director of the Stanford Center on Longevity in California, observed that people tended to interact with fewer people as they moved toward midlife, but that they grew closer to the friends they already had.