There are different metrics to measure social media – and there are metrics delivered by social media monitoring tools which is something different. This means social media activities are not measured by social media montoring (tools) alone. Monitoring tools deliver important insights and metrics alongside with well known disciplines like controlling or market reserach.

Unfortunately there are no standards to measure social media, yet. A good summary of the current stage of setting social media measurement standards is provided by “The measurement standard”:

If there are no standards and everybody is measuring what he wants, the only possible conclusion is: Changing the tool changes results.
And: Results of one company are not comparable to that of another unless it is measured with the same tool.
Is this a reason not to monitor social media: No! It is just an insight which has to be considered when talking about and comparing monitoring results.