Try to explain the benefits of a smaller chapter. For example, pnm's who are leadership oriented might like the idea that there are more oportunities for leadership positions.
also, stress the sisterhood that you share (which you should always do!). However, with a small sisterhood, you can stress that it is a family and that you truly know each other.
You said that you are the last active chapter of you sorority. if you have plans to try and expand again, this could be a great selling point. (Not that you are the last chapter but that a pnm could have the opportunity to essentially refound a sorority. I know that would have impressed me as a "rushee"
Good Luck and Greek Love!