Factors that influence engagement with Facebook wall posts

"We looked at three primary success metrics in relation to Wall Posts:
Comment Rate: number of comments as a percentage of fan base
'Like' Rate: number of 'likes' as a percentage of fan base
Engagement Rate: a combination of the above factoring in fan base size
Key takeaways from the research include:
Post Timing (Hour): Brands that posted outside of normal business hours had 20% higher engagement rates on their posts.
"Promotional" Keywords: Which keywords are most engaging when running a contest, sweepstakes or other offer?
Post Length: Posts between one and 80 characters had, on average, a 27% higher engagement rate than posts with over 80 characters, yet accounted for only 19% of all posts.
URL Shorteners: Engagement rates are three times higher for posts that used a full-length URL, as opposed to a URL shortener."
Source:  Research by Buddy Media, reported in their blog, 6th April 2011