I noticed that a good portion of jumpseaters ignore the FO's. It never really bothered me when I was an FO, half the time I would not even turn my head to see who wanted the jumpseat. Would not want to inflate their mainline ego any more than it has to be.
Just as a side note, a few years ago I was returning home from a job interview and jumpseated on United. I approaced the Capt with my ID, jumpseat form, still wearing the ultra conservative interview suit and tie. When I politely asked for a ride home the crusty old captain responded with "Do you know the rules?" to which I responed "yes sir" he then offered a seat in the back. I guess he thought I was going to grope the flight attendants, or smear feces on the walls or something. Since then I had always wanted to see this guy asking me for the jumpseat. Instead of a simple "certainly", I would of course say "Do you know the rules?"