I have just been reading the 2015 Sensis Social Media Report, which contains all kinds of interesting facts. For example, out of Facebook, LinkedIn, Instagram, Google+, Twitter, Pinterest, Snapchat, and Tumblr, Facebook is the most heavily used. Also, the cliche “Men are from Mars, women are on Pinterest” turns out to actually be true:
Other data shows that Facebook usage is trending slightly downwards over time, while use of LinkedIn, Instagram, and Google+ is increasing. After rising up to 2014, Twitter usage is also down. Consistent with this, many Twitter users report deleting their profile. To a lesser extent, Facebook and Snapchat users also report this.
Data on age shows that under-30s are particularly active on Instagram and Snapchat:
Update: For comparison, here are corresponding charts for the USA:
Data for these second two charts is from a report by Pew Research.