I'm not the best placed to share idea about this kind of feature : I'm not a social internet user (I don't use twitter, no facebook, no google-truc, ...)

But some rough idea, for technical point of view:
- using a plugin is good idea : it's a good method to see if people are interested or not;
- referenced site can be centralised on a server (an url like http://network.piwigo.org could be a good thing); access to this page from a webbrowser allows user:
      * to see how many galleries are referenced (it's public, and it give visibility for non piwigo webmaster's users)
      * choose to visit a random choosen referenced gallery
      * choose to visit a gallery from country (french, english, japanese, spanish, ...) or keywords
- to be referenced, webmaster install a plugin:
      * allows to choose to be referenced or not;
      * allows to add small description of the gallery (1500char for example)
      * allows to set from 0 to 5 choosen pictures on the gallery (needs to be a public picture !)
      * can be referenced only if more than 10 pictures are public
      * allows to choose from predefined keywords list (loaded from server, not hard coded) the themes of the gallery (ie: nature, architecture, people, cities, mountains, erotic, wallpaper, astronomy, birds, ....)
              => managed on server, this permit to the webmaster to choose keywords in its own language, but displaying it in the visitor's language
- and the plugin can used just to add "external album"
- I'm not sure, but I think it's possible (with twitter & facebook) to share link using specific url: http://see.piwigo.org/1234578d redirects on the gallery url
- on the server side, implement something doing one time per day :
      * check availabilty of url (if url not responding, retry in 1 hour, if not responding, retry in 2 hours, in 4 hours, in 8 hours, in 24 hours, after : disable the gallery from the server (can't be viewed by visitor) and send a mail to the webmaster ; retry 2days, 4 days, 30days : after, definitively remove the gallery from the server)
      * retrieve the number of picture (with date of the last picture and number of new pictures from the 7 days)
      * do to this, the plugin have to implement webservices

mmmh.
finally I have some ideas ^_^;
probably if I continue to think about it, I could find another stuffs...