This entry is part of our ‘Social By The Numbers’ series which features weekly content focused on social marketing data analysis, insight and trends that marketers and business intelligence managers can use to better engage with their social communities. Look for a new post from Fred Stuk and the Vitrue Business Intelligence team every Tuesday at 10am EST.

In previous Vitrue blogs, we gave tips on how to best optimize your tweets.  One piece of advise was that just because you are allowed 140 characters does not mean you should use every one of them.  Eyes are drawn to copy that can be read in an instant.  Plus leaving spare characters leaves you plenty of room to get retweeted with a comment.  Clearly, Twitter is a social network designed around brevity.

But what about Facebook posts? Is there an optimum length for posts?  Do long posts turn people away or make them less likely to engage the content in some way?  Are posts that are too short (i.e. a short link only) too suspicious to get clicked?

We looked at posts made by the 150 largest Pages on Facebook to determine if there is an optimum length for engagement.  These Pages represent businesses, industries and people of all types; brands, musicians, politicians, celebrities, anyone with a Page worth talking about.

Thanks to the handy dandy Facebook Graph, we were able to collect all of the posts, plus the Like and comment count associated with each of these posts over the last couple of months, August 1 through October 10, 2011.  Our collection wound up encompassing over 11,000 posts.  Taking the Likes + comments and dividing by the number of fans gave us the engagement rate as a percentage.

We looked at the character length of our sample posts up to 240 characters long.  240 characters represented the highest meaningful post size.  While there were indeed posts that went above and beyond 240 (some all the way up to 600+), they represented such a small percentage of posts in the sample that 240 were a reasonable point at which to carry the graph.

From the above graph, we can see that as posts get longer, the engagement rate decreases steadily and in step with the rising number of characters used.  There does not appear to be a point beyond our 240-character limit where this trend reverses.

Short posts do not seem to arouse much suspicion or get passed over merely as a function of being brief.  For example, the soccer team Manchester United put up a post that said, in its entirety, “8-2…” It was nothing more than the score of a match, but it garnered 173,000 Likes and 29,000 comments.

Does this mean that you should strive to keep your Facebook posts a mere six characters long?  Not necessarily.  Marketers must also take into account what type of post they are putting up, and how that particular kind of content affects engagement.  Whether a post is text, image, video, poll, or game absolutely has a direct bearing on engagement potential, as we at Vitrue have pointed out many times in previous blogs and white papers.

The key takeaway: There are measurable advantages to keeping Facebook posts concise as they can be, while still fully communicating what you want.  As with Twitter, Facebook posts that are a quick read, easy to grasp and can instantly be acted upon, have a higher likelihood of being engaged by users.