MySpace may be experiencing the greatest decline, but unique user visits are flat or declining slightly across most major social networks. Twitter, Facebook and MySpace are each aggressively rolling out new features, but none have yet had a major impact on traffic.
Don't extrapolate from this data that social networking is dieing. Rather,it may have just reached a user saturation point where any further growth comes more slowly and at the expense of a competitor. For now, however, mass social networking is a still three headed monster and all three mouths need to be fed.