> Doesn't that depend on how you're defining efficiency?

Yes. But I think that's why you have to be careful. Common-language use of the word "optimal" is very different from the technical meaning of "Pareto optimal", but its very easy to glide from one to the other.

The example wiki gives clarifies the situation: suppose someone has a monopoly. Getting rid of that will benefit all of society - except for one man. But if you're constraining yourself to Pareto optimisation, you can't do it.