After reading McGrenere and Ho, I was able to differentiate between usability and affordance. I think the term usability deals with user’s experience with a system, while affordance deals with the properties of the tools that are utilized to design a system. For example, a blog might not be usable (usability) if “comment” is hidden whether by its location with the post, colour, or/and size. In the case of affordance, the designer of the blog is availed of the tools, i.e. comments, search, blogrolls, tag cloud, and many other widgets, in blogs.
The FVRL maintains a Youtube account that helps both librarians (public library especially) and the public. The videos are educative and serve as resources for librarians of public libraries, especially. Posting comments in a similar fashion as blogs is a good prove of its affordance as a social media tool. Comments in YouTube are only textual. I think this is a limitation to YouTube affordance.
Some people prefer audio/visual to textual communication. VoiceThread gives one the opportunity to comment on posted video (or multimedia file) in different formats other than typing the comments. Thanks to Dean for making reference to VoiceThread in one of the class discussion forums. Its About page is a starting point to know more about its affordance.