I'm sure FourSquare could probably write a long explaination of why they decided to migrate, but simply put, the tl;dr version could be summarised as: "We can'

I'm sure FourSquare could probably write a long explaination of why they decided to migrate, but simply put, the tl;dr version could be summarised as: "We can't sustain a $8 CPM hit." (I'm assuming $8 since they use custom styling). Most sites can't even sustain a $4 hit. If I could guarentee $4 per 1000 visitors to my site (income through advertisements) I would be a lot richer than I am today.