Almost nine in every ten U.S. companies of 100 employees or more will use social networking for marketing purposes this year, suggests a new study from eMarketer.

This represents an eight percentage point gain on 2011, and a leap of some 31 percentage points since 2009, to current figures of 88 percent.

The problem? With so little upside left, eMarketer expects growth of just three percentage points by 2014.

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