Your social meida footprint: fans/followers. Fans are a "reach" which translates into the number of people you can reach when all your posts are seen by all your fans. You target along their extended networks.
Networks are fast-paced. The reach is expected to go down! Not all your followers see your updates. Metrics from different networks will give you the data you need to see who you are reaching.
Metrics to consider:
Organic impressions [Facebook impressions]
Google+ Followers [Potential Reach on Google+]
Twitter Followers [Potential Reach on Twitter]
Twitter Reach [ Via Twitter Analytics]
This way you can find your brand reach and make meaning from Big Data.
Marketing strategies should be measured. Measuring social media is trciky, with no one particular way of doing it. Consider that. Consider to create your own system.
Concentrate on what matters.