A song of Ice and fire discussion (spoilers incl A Dance with Dragons)

['thul]

these beings just came to realize something in ASoIaF...

Spoiler

The wall is the single most important structure in the realm, it is clearly ignored often, generally to the despair of all when the walkers come. The citadel of the Maesters is known to send out the white ravens upon the turn towards winter. Why does not the wall have white ravens or such messengers? They should technically have had a set of special messenger birds, both to send to Winterfell and to send to the citadel for further spread of warning upon the tripled call of the horns.
Is it because they come so rarely that the system is missing, or is it because it was abandoned over the eight thousand and something years since they last came?