Impression is the same number as potential reach on the PostBeyond platform. It means the combined number of followers/friends/contacts you have in your social network.
The term "impression" is used when you set up your Earned Media Value settings and Leaderboard settings. In both cases, the number of social contacts is divided by 1000 for the calculation.
For example, if the "Twitter impression" is set to 1 under the Leaderboard settings, this means that per 1000 Twitter followers, users get 1 leaderboard point.
If the "Twitter impression" is set to $1 under the Earned Media Value settings, this means that per 1000 Twitter followers, the company's Earned Media Value on the Dashboard will increase by $1.