Oh dear, I could see this becomming a major problem for those with FB pages!
Usually it doesn't matter if one lunatic customer gets upset because you just drop them and move on. They don't know your other customers so no matter how crazy they are there's nothing else they could do.
But now by encouraging customers to like your facebook page, you have brought them all into one place to have a chat. BAD idea.
Personally I just don't see the point of using facebook for window cleaning. Yeah you might get the odd referal etc, but I imagine that would be relatives who don't necessarily live in the same area. I'm not friends with any of my neighbors on FB, nor do I suppose most people are.
If you need to communicate with your customers, eg to offer conservatory roof cleaning etc, you can send them an email etc or better yet give them a letter when you're there next, you don't need to use facebook.