I'm not really Facebook clever, and have used my page in the past to advertise, but why would it help to have window cleaners as members? I've picked up a lot of work in the past by posting directly on other groups, telling them who I am and where we work etc, but otherwise, who reads what you write on your page. I don't read what our plumber writes on his page. 
I bet I've missed the point here. Sorry if that's the case.
Today alone, I have had 6 facebook customers, all 6 of which have been booked in.
The advantage of having other window cleaners like your page, is so that when you upload photos of your work etc, the pictures will get likes, and comments which complement your work for others to see.
Oh... and its free advertising
