Facebook Invitation Etiquette

So it was sort of obvious that when a former student of mine invited me via facebook to an end-of-semester party at at off-campus apartment, that the best thing to do was to politely decline.

Now I've been invited by a different former student (same class) to an on-campus event being organized by an official student organization to raise money for a bona fide charity. I'm guessing that she just clicked the button for inviting all her friends. Yet it does raise the question of what the etiquette is in these situations.

I continue to politely decline to my independent study student's repeated invitations for me to go out swing dancing. I don't dance; I never have danced; I never will dance.