It's not always the case but, in general, satisfied customers will say nothing - but dissatisfied customers will be more vocal on the internet. One of the problems is that somebody can be dissatisfied because you won't adapt to their individual needs (e.g. only clean if I'm in), but they won't actually say why they are dissatisfied.
Result: You can end up with dissatisfaction being expressed but have no way of answering it.
My take is that I prefer not to give customers that sort of power.
I won't adapt to your needs?
No problem, find a window cleaner who will. For pity's sake, if you're nice about it, I'll even help you find one.