As social media platforms like Facebook are being used by advertisers to reach millions of users, it is critical to accurately represent the impact of posts/comments on Facebook.
Yes, there are 750MM+ unique users on Facebook, but not all of them are active at any given time. Thus, when a comment or post is pushed on your company/brand's Facebook page, not all of your Fans see/interact with them or have the opportunity to see/view them.
According to sources at Facebook, only 5-15% of the fans view the post in their newsfeed. So, if you have 100K fans, only 5-15K of those fans will actually view your post.
The main reason for this dramatic drop off is the fast moving news feed. Typically, a post is active only for about 30-45 minutes.
Also, the EdgeRank algorithm that Facebook uses keeps into account the Affinity, Weight, and Time to pick the stories/post that appears in a user's news feed. So, the more a user or fan interacts with a brand's facebook page and the more fresh/updated the posts are, the more changes of the posts to show up in fan's newsfeed.
It is estimated that this percentage is even low for Twitter as it is even fast moving. Typically, 5-7% is considered as industry norm for Twitter posts or Tweets.
So, please be careful in representing the opportunity to see/view impressions.
If you have 100MM fans and your brand posts a story on Facebook – approximately 5-15MM users will view the post.