You really don't understand anything about social semantics, network theory, and just basic human behavior. If FB continues to exploit the fundamental behavior of human/social networks, FB will inevitably change the web as we know it. You are thinking in terms of product or service life cycles, however FB's core or fundamental purpose/advantage spans beyond mere product or service life cycles. Just take a look at how many companies try to exploit social networks in their services. This has been going on for decades. Even before the advent of the internet and computers, there have been social networks that people thrived on and reaped great benefits from. Everywhere you look there are social networks. However, FB succeeded in defining it and making it purposeful.