As the world's population increases and more people join social networks, how much are brands' follower numbers growing? What qualifies as a "good" social media audience growth rate?

To find out, TrackMaven examined full-year 2015 social media follower data for 26,965 brands across a wide range of industries. The researchers determined the median monthly follower growth percentage per brand on Facebook, Twitter, Instagram, LinkedIn, and Pinterest.

Brands' median annual follower growth was 42% last year across the five major social networks examined. Put another way: a typical, middle-of-the-road brand had 42% more followers on Facebook, Twitter, Instagram, LinkedIn, and Pinterest combined at the end of 2015 than it did the beginning of the year.

The highest growth rate was for Instagram, with the median number of followers increasing 6-8% month over month. Annually, brands' median growth in number of followers was 100% on Instagram in 2015.