Dive summary: 

• A study released from Facebook states that, on average, each Facebook post is seen by about one in three of a user's friends, or approximately 61 percent. 
• User perceptions were also measured and results showed that users "consistently underestimate" their audience size. 
• The study will be presented in full at an unnamed academic conference next month. 

From the article: 

"On average, each post was seen by one in three Facebook “friends,” according to an analysis of 220,000 Facebook users’ posts last June. Over the course of that month, users reached an average of 61 percent of their friends."