An impression happens every time a user sees any post from a Page, anywhere on Facebook.com or in a Facebook mobile app.
Technically, an impression is counted whenever a status update is rendered in the browser. So if someone visits their Facebook newsfeed or your fan page wall, then Facebook counts impressions for all the posts that they send to your browser, even if the user doesn’t scroll down the page to see them.
Facebook reports impressions in three ways:
The total number of times that users have viewed this particular post.
Counts the total impressions generated on a given day across all your posts for a single page.
Counts the unique impressions generated on a given day across all your posts for a single page.
A pageview is counted when a user visits your fan page at facebook.com/pagename
Each tab visit is also counted as a pageview, so if a single user visits three custom tabs, it will count as three pageviews, but only one unique pageview for that day.
Facebook reports pageviews in two ways:
Counts the total pageviews generated on a given day on your fan page, including your custom tabs.
Counts the unique pageviews generated on a given day on your fan page.
This is a breakdown of the number of Page likes from the most common places where people can like your Page. Note: this does not include all areas of Facebook where users can like your Page. (Unique Users)
The number of times people (Fans and non-Fans) have viewed a News Feed story posted by your Page.
Daily Top referrers to your Page on Facebook (Total Count)
Daily Top referrering external domains sending traffic to your Page. (Total Count)
The number of people who have interacted with or viewed your Page or its posts. This includes interactions from Fans and non-Fans. (Unique Users)