Actually, this is what Reyes referred to as the "law of ver large
numbers" in that o-so-painful earnings release two years ago.  Google
cannot sustain the same rate of growth indefinitely because the
current pie has a limited size; you can only grow at 60% for so long
before there is no more room to grow.  It's not going into negative
growth but growth rate will continue to decelerate unless they start
expanding their pie.

You are right, this is why they are venturing into social networks,
mobile network access, foreign markets, and any/every other area.
Their hope is that they can effectively monetize all areas that can be
searched.  This will always be a problem for Google as long as they
expand in double digit growth.  They will eat up the market as they
expand and will need to find new territories.

A good example of this is the Cola Wars between Coke/Pepsi.  They
fought until they reached a stalemate in cola markets share, then the
companies stagnated until they moved into fast food, then lately, the
move into water.  Same thing... you can only have hyper growth in any
market for so long until you either reach 100% or stalemate with your
compeititor.