These fares are limited to just a handful of flights in each city pair, pretty much all on off-peak days.
Seat maps on the MKE-FLL-MKE flights with the $49 fare suggest these were booked to an average of about 55% full. I checked first thing this morning and hope to see how much the seat maps change after the sale.
Seat maps on the A319 flights are somewhat less reliable to guesstimate load factors because more people appear to book without seat assignments. If you compare the seat map of a YX A319 flights 25 hours and then again 1 hour before departure, the number of occupied seats tends to jump a lot more than seat map of an E-jet or RJ flight. But keeping that in mind, I'd guess the select MKE-LAX and LAX-MKE segments with the $69 fare were booked mostly in the 40-50% range...definitely among the weaker days. I agree with newsmanhoss' notion that MKE-LAX is overserved for the time of year with 3x day, although the connecting traffic coming through MKE makes a difference when it comes to filling seats. From watching seat maps I'd estimate MKE-LAX for Midwest to have averaged around 70% for January, which isn't bad considering everything. But January has days which benefitted from the holidays and the MLK weekend, and there are some flights running quite a bit lighter than that.
Finally, MKE-EWR in general seems to be suffering in light of the high capacity low fare service to LGA. In addition to YX just having the 2x/day ERJ at this point, Continental keeps pushing back on the market, too. For a couple of years Continental has cut MKE-EWR down to 3x/day in January, but usually went back to 4x/day by mid February and often 5x/day by spring. That's how they originally had the 2010 schedule loaded, but they have at least twice now delayed the restoration of flight #4...now it doesn't come back until June...and there's no sign of #5 coming back at this point. One wonders if Midwest will keep MKE-EWR as a low-capacity/higher-fare market for those people who will pay more to get EWR, or if they will operated larger aircraft there for lower costs/lower fares to relieve pressure on the LGA flights. The 5x/day MKE-LGA flights are filling the E170 pretty well in the dead of winter, and more seats to the NYC market will undoubtedly come one way or another come warmer weather.