Here Facebook grabs the Mediawiki logo instead of the site logo.
http://www.facebook.com/groups/tg.taiwan/permalink/374509135949001/?comment…
Doing the same experiment with e.g.,
http://en.wikipedia.org/wiki/1st_clan_chief ,
a page also without any user embedded images,
oddly does not cause the mediawiki logo to be chosen.
Though it does not choose the site logo, at least it doesn't choose the
mediawiki logo.