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One of the wonderful things about social media and the digital age is that results are so easy to measure. We can see moment-to-moment how many likes, followers, hits, forwards, and retweets we get. As long as you set good measurable objectives early on in the process of planning, you can easily measure whether you hit your goal.
Without spending much money, you can measure traffic to the platform you are using and pinpoint specific times of day traffic is either heavy or low. If participation goes way up during booth hours, you know your instructions and promotion are working. If you get people signing up for your contest before the trade show even starts, you know you are doing a good job promoting it. Tools are available that allow you to track who is spreading the word for you. Some are free like Google Analytics; some carry a fee, like Radian6. You can even set up a reward system for those people who are sharing your content.