Are you looking at just one company? You can use likes, check-ins, retweets, hashtags, followers, fans, number of friends of individuals who are friends or followers can be calculated to determine potential reach, semantic analysis of comments, click throughs, and for Youtube views and comments,
For twitter there is a tool called Tweetreach. Facebook Insights is also a tool.
Then combine with a survey delivered via social media
Consider hiring a PR firm to do the work for you...
Reputation is a very subjective topic. Looking at Likes or Share doesn't mean anything. if you are trying to evaluate whether your product or service are popular amongst your consumer, measure sales, repeat purchase or even referral (if you can).
If you really want to do this through social media, I would suggest to prepare a questionnaire through Survey Monkey (for example) and propose an incentive for user to fill-up your questionnaire.
Last option, you can still use Google Trends to measure how your topic is being discussed.