According to eMarketer.com’s recent study, having a picture and a clear call to action (e.g. request for likes or comments) leads to the most engaging social posts: http://www.emarketer.com/Article.aspx?R=1008677. My own research by running social campaigns for 6 Fanpages simultaneously has led me to agree with this, except for 1 additional comment: Keep it short!
Keeping your posts short, and varying the time of their postings (possibly using my companies own tool: postontime.com) is the way to go! Shorter posts are more likely to get read, and short posts with calls to action (“Like this if…”).
For more tips on making your social campaigns work, check out my white paper here.