If ever you needed proof about the differing age profiles of Facebook, Instagram and Snapchat users here it is:
The data is from USA, but the picture is clear. The 18-24's are definitely moving from Facebook to Snapchat and the 25-34's are more heavily using Instagram. Of course, Facebook has over 1.5 billion users, so the age profile reflects the total online population and is therefore more spread between the ages, whilst Snapchat is newer and has fewer users who are almost exclusively under 34.