Poll is flawed. Ideally (if there is another person there) you should do all three of those, or at least call patrol and send someone for help.
The first, best thing to do is to call ski patrol (or the ski area's main number if you don't have the patrol's direct number). As mentioned above, it helps a whole lot to provide accurate information as to where the victim is, especially at a larger area or if you're off-trail.
If for some reason you can't get ahold of patrol, and someone is seriously hurt, calling 911 works too. They should be able to get the ski patrol on the line. As noted in posts #19 and #20, this is not ideal for various reasons, but it's WAY better than doing nothing.
If someone else is with you, you should send someone to go to the nearest ski lift (or the base area) and have them notify a ski area employee about what happened and where the injured person is. All lift operators and many instructors or other on-hill staff will have a radio or phone to call ski patrol.
I would not leave a seriously injured person alone unless there was really no other choice (e.g. nobody else there AND no cell/radio service).
Crossed skis (or a snowboard placed sideways as a barricade) in the snow above the victim will draw attention and hopefully keep anyone from crashing into you if you're somewhere not easily visible from above.