That's perfectly fine thinking if it's something you found yourself. However, implicit in linking something on a social link aggregator is a sort of "here, read this, it's good" on the part of the poster, which just doesn't jive with the site asking for a fee. It's like bringing donuts for your friends, opening the box and telling them you got each person their favorite, and then charging for them—it's just not how our particular social machinery works.