Facebook first came out in 2004 and you had to be a college student to use it, so I think it makes sense that it stuck with adults for the most part. Facebook did have a time period where it was very popular with teens but Twitter took the spotlight from them beginning in 2006 and the simplicity of tweeting was more popular with younger people, where they didn't have to read all these long posts by adults on Facebook. then in 2010 along came Instagram and teens loved it because, well, we're raised in a narcissistic culture and how better to exacerbate narcissism than a social media website designed for posting pictures of yourself. then in 2011 snapchat made its debut, with its primary purpose being to send people your private pictures without it being permanent (hence the timer on a picture). while people not only use snapchat for that purpose anymore, it's still a popular way to use the app.
tldr: there are three main themes of the current generation which are shortened attention span (Twitter), narcissism (Instagram), and s3x (snapchat).
in my English class last year I once read a piece about how our attention span is really short now but I forgot the authors name, so I went googling and found something similar here:
http://nytimes.com/2016/01/22/opinion/the-eight-second-attention-span.html
