Each day, every person who clicks on the link adds towards increasing the population of the city. Only one click per person per day is counted though, so I think I'm going to be clicking a lot at work as well.
Once the population reaches a certain level, then another link becomes active to increase the industry of the city. Only one of the two links can be clicked per person per day. Then it adds transport, security, environment and business.
They don't ask for email addresses or anything like that, it's just the shared links. Nifty, and pretty too.