Measuring your influence can be a good way of gauging whether you are talking about the right topics, influencing the right people and growing your network. Facebook and Twitter have built in analytics within the website. You can find out about the demographics of your fans, your top performing posts and your engagement rate. By understanding what works and what doesn’t work for your social media account, you can steer your future messages.
There are also other websites (some for free and some with a charge) that will give you more information e.g. Falcon, Tweetdeck, Hootsuite. By giving these websites your account details they will calculate many metrics for your social media accounts.