Agreed. If Google's incredibly talented and distributed employee base can pull their networks in to whatever their new social initiatives are, it's a huge aset to them in the battle with Facebook and everybody else for "who owns social". Tying everybody's bonus to this encourages each individual employee and team to think creatively about how they can help. It's a strong signal that "this is our top priority and yes we really mean it".

Which doesn't mean it will work, of course; see ScottBurson's points above. But from a strategy perspective it's a classic way to align priorities around the critical corporate goal.