Some of the posts on my Facebook wall have a 'share' option and some don't and I assumed there must be a setting in the 'Privacy' options that would allow me to set the 'share' option for all posts.
Well, Peter 'Toxane' Michael was kind enough to put me straight on the issue. He advised as follows: "Wall posts with attachments (Links, Notes, Photos etc.) can be shared, 'text only' posts not. There is no setting for this, it's a 'feature'. ;-)"
I'm not sure why Facebook make it more difficult to share text but it's a limitation you should be aware of, particularly if you are promoting a product or service.
The bottom line is if yoiu want a post to go viral then include an attachment.