Social sites change with age?

When looking into the idea of social sites and how I personally use them I realised that the available sites all have set age groups that use them and as you grow up you progress to the next, most following the typical pattern of bebo to myspace to facebook. The future development would then most likely progress to more adult based social networks such as friends reunited. It also came to my attention that often we do not join social sites such as those we are making which are subject based and merely just to follow the trend. Therefore the sites we are designing could have a future to grab a niche market, which as such is not being promoted. What if google instead of competing with facebook explored different social networking and brought something entirely new to the table.