Only 6% of fans engage with Facebook fan pages in 8 weeks

"Most brands understand the importance of an active, engaged Facebook community, but few brands know how to build one. Fan participation is never strong enough, so brands do what they think makes sense: focus on getting more fans, and posting more content. But today, Napkin Labs is turning common sense on its head. A study conducted by Napkin Labs reveals that when it comes to Facebook, a few very active "Superfans" are way more valuable than the overall quantity of fans.
Fan Engagement: Quantity Does Not Equal Quality
For the study, Napkin labs analyzed fan engagement across 52 Facebook brand pages, with each brand page having between 200,000 and 1,000,000 fans. Of the more than 31.7 million fans who were analyzed over eight weeks, only 6 percent engaged with a brand's Facebook content. While most brands think the best way to boost overall participation is to accumulate more fans, the study proves this is untrue. As a brand page's fan count increased, the amount of engagement from fans didn't necessarily increase along with it. For example, brands pages with over 900,000-1,000,000 fans had 60 percent less engagement than brand pages with 500,000-600,000 fans."
Source:  Press release from Napkin Labs, 18th October 2012
Note - Clearly brands know this, and much of what brands and agencies now do on Facebook is aimed at driving engagement among the fans