I, on the other hand, put down
weed & feed twice a year, overseed, and apply lime when necessary.

So guess whose lawn has been taken over by crabgrass?

weed and feed does not kill crabgrass. It only kills broadleaf weeds. Has it
been hot latley where you are located? If so and your lawn is a cool season
grass, fertilizing in the summer wont help your lawn, and actually can help the
crabgrass(a warm season grass).

If so does this mean the whole weed & feed concept is a
farce?

No, you just have to understand what herbicides work on what kind of plants.
Grass plants are usually not affected by "weed and feed" or "lawn weed killer"
products. You can apply them all you want and they wont touch your crabgrass.
There is really not much you can do now other than keep up on your mowing and
make sure that the crabgrass dosent flower and set seed for next year.

Toad