I can only offer my recent experience to this question, our flight to DFW from PDX on NK was 85% full, the return 90-95%, the boardings in DFW we witnessed on NK all seemed to have a lot of passengers. So my guess is they are filling planes, but if that equals a profit or loss I don't know.
I will say, NK is not for everyone, and certainly not anyone who is snobbish in any way about flying, they do seem to attract a level of clientele that I have never encountered at 30,000 feet before. NK creates demand in a market, the few people on my flight I spoke to, all said they would not have otherwise made the trip had it not been such a low fare. All were O/D traffic too, hardly nobody seemed to be connecting, as all filed out past security upon arrival into DFW.