Humans can only process so many friends - around 150, according to anthropologists. The rest is grooming, like chimps. And the real circle of communicants is stable and small:

[A]n average manone with 120 friendsgenerally responds to the postings of only seven of those friends by leaving comments on the posting individual’s photos, status messages or “wall”. An average woman is slightly more sociable, responding to ten. When it comes to two-way communication such as e-mails or chats, the average man interacts with only four people and the average woman with six. Among those Facebook users with 500 friends, these numbers are somewhat higher, but not hugely so. Men leave comments for 17 friends, women for 26. Men communicate with ten, women with 16.