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Evaluating the quality of a social media consultant or manager requires an assessment of the results they have consistently delivered across multiple social media campaigns. While many businesses (and consultants) measure social media effectiveness solely on activity-based metrics such as likes, follows, etc, more concrete social media ROI measures can be applied to social media campaigns with disciplined tracking of leads or support requests from social media sources through to sales or issue resolution. These measures can include conversions of social media activity into sales or tracking the cost of supporting customers via social media compared to your traditional means of customer support.
A qualified social media consultant will be able to provide you with reference clients and the results of each of their social media campaigns, so that you can objectively review the actual results of their efforts in assessing the effectiveness of the consultant (you should also get to speak to some of these reference clients in order to validate the campaign results). Comparing the outcomes produced by various social media consultants (and seeing the process each one uses for measuring success) should enable you to identify a consultant that meets your needs.