While DiiGo addresses finegrained annotation on arbitrary pagepositions, WikAlong only builds coarsegrained pagecomments. On the other hand WikAlong is powered by an underlying WikiEngine, whereas Diigo is on its way from originally poor text notes, via Wysiwyg rich text notes to a fullfledged WysiwygWiki.
Discussion: The author pointed to here from his new [SeedWiki:SlideWiki:SocialAnnotations], because the topic has a broader application in the general community. The special application for WikAlong in a SlideWiki is to source ThreadMode out of the base wiki and concentrating only on DocumentMode.