I have to wonder how much they'll use snapchat once they are married and have kids. By then, there'll be a new generation of young people that'll want their own app to use. That was the issue with myspace. It was targeted at a very specific age group. When you do that, you're usually only popular for a short time period.
There's a reason music artists targeted at that age sell the most their 1st album. The 2nd is usually 50% less, then the third is 50% less than that, and by then they are irrelevant. The new group that is that age wants their own musicians to like.