There's a recruiting post (see: http://news.ycombinator.com/item?id=3881446) on HN that mentions Socialcam gained 4M users this weekend. How did they do it?
Alexa: http://traffic.alexa.com/graph?w=400&h=220&o=f&c=1&y=t&b=ffffff&n=666666&r=6m&u=socialcam.com&
