3G Internet Not Working

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http://androidforums.com – I've only been using wifi ever since I got my phone (I changed my settings from 3G to wifi only). I bought it unlocked and switched the network from Vodafone to Optus. I've just switched recently to the new Optus Prepaid Social plan, which gives me unlimited access to social networking, but now my internet isn't working at all. The wifi still works but once I switch from wifi back to 3G, my Facebook, Twitter, even the browser doesn't work. Someone please help. (General)