SIMPLE BUT SOLID ANALOGY.
"Is it just me, or is there a strange similarity between all these social networking sites and say, areas of a city that get hot with all sorts of bars and nightclubs and then go cold in a matter of months? I mean, Friendster was huge, then it wasn't. LinkedIn got big, MySpace got big, and now Facebook seems to be getting big. Someday, Facebook will be cold, too. Just like neighborhoods or nightclubs."