A friends feature does exactly what you'd expect it to, telling you who is online and who isn't. The City Log side of this tells you what the online friends of your list are up to. In small, automatically updated boxes not unlike those of a Twitter feed or Facebook updates, expect to see updates on new leaderboard rankings, new goals reached, or open invites to play along with someone.
Given that all sessions of SimCity are online and always connected, Maxis can look at the world and see overall changes and trends, and then use those to issue challenges to players. The Challenges feature lets you get in on these creations, and you'll win rewards for meeting their requirements.
For example, imagine if Maxis saw that there was a high unemployment rate for the entire SimCity world. They could issue a challenge that would have everyone working together to solve the unemployment problem, and award all participants with an achievement. Or, say that the worldwide population was down. A challenge might have regions competing to see which could reach 1 million people in the fastest time.
The Global Market feature tracks the entire collective economy's resources and displays them so that they can be used by the player base. Resources like ore, metals, coal and more are trached in real time. SimCity has a real, functional economy, and supply and demand are simulated, so players can track this data and take advantage of it. If the oil demand tracked high, a new player could create an oil-based economy for their city to take advantage of the demand, for example.
Finally, Leaderboards are a given for an online game, but the scores that SimCity tracks are a bit different from what you might expect. One example showed a pollution leaderboard, with the 'winner' being the city with the most pollution. There are several categories that go beyond money and fame; Healthiest, Greenest, Sickness, Crime, and Tourism were among those listed.
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