Whereas each lifecycle has its peak, the latest study by PEW Internet has some surprising results. For the first time in the brief history of the social networking era the overall usage of social media decreases among internet users from 69% to 67% and this within a period of time generally known as a period strong usage. It is even more remarkable that the youngest age group surveyed has the highest drop from 92% to 83%. Only within the age group 30 to 49 the adoption increases. Meanwhile on the mobile internet 40% of internet users access social media sites and 28% do so each day: Pew Internet: Social Networking (full detail).
Furthermore some facts about the demography at particular social networks are available by another study: Popular SocNet Sites Prove Appealing to Varying Demographics.