Impressions is the number of times your ad was viewed and included social information. Social impressions may be different than the number of unique people who viewed your ad. For example, if the ad is viewed by 3 people 2 times each and it includes information about a friend liking your Page, that means you have 6 impressions.
On the other hand, reach includes the number of people that see your content. When we are reporting “reach”, we are looking at how big our effective audience is. This means if the ad is viewed by 3 people 4 times, your reach would still be 3.