The individual data plugs are built around different data sources that are used to pull in data. In this case, Photos Plug is built around Dropbox and can be used to pull in photos stored in various Dropbox folders. Similarly, Social Plug is built around Facebook and currently supports data categories such as posts, events, profile, profile picture and music listens. However, at this point in time it does not support facebook photos.
Facebook photos would be a great addition to the Social data plug and we are hoping to add this data category very soon.
I hope my explanation helps with the confusion. Any ideas how we could improve the interface to make things clearer?