Depending on what apps you have installed, apps like facebook, twitter, google+ and others will download data in the background to keep current when you open the app.
With ICS, you can disable background data usage for specific apps:


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this is visible in the system settings -> data usage. you should then see a list of apps that are using data. it will also show the highest usage app. Clicking on the app brings you to a details screen that allows you to disable data usage or background usage.