User Activity Comparison Of Popular Social Networking Sites [Infographic]
Viktoria Ruubel stashed this in Social Networking Stats
Thank you for stashing this, Viktoria!
So a good growth hack is to establish a symbiotic relationship with one of these networks.
Also, here's an interesting infographic on what consumer Internet users are worth: http://pandawhale.com/convo/3386/whats-a-consumer-internet-user-worth