Free access to social networking sites has been used as a carrot to attract mobile users before, but 3's latest deal is pretty comprehensive: free access to Twitter, Facebook, LinkedIn, FourSquare and MySpace for contract mobile customers.
As is invariably the case, the devil is in the detail: you need to be on a contract plan, and if you're an existing 3 customer you need to have renewed that contract after July 1 this year. Whether getting free social networking is worth two years of not being able to switch carriers depends on your usage patterns.
If you're an iPhone user, it would certainly make sense to wait until new iPhone 4 plans are announced. Telstra's Tribe offering gives similar free access to Twitter, MySpace and Facebook for Next G customers, though you have to use the specific Tribe site.