I'll be honest lads, Facebook isn't the best place for window cleaning. How many people think about window cleaning while their on facebook, or want to connect to a window cleaners page.
You're never going to have great success from it. It's not that kind of business (unlike clothing labels/food shops etc).
The only way i can see to get better engagement is to offer little tutorials or tips on things people can try at home themselves (some window cleaning/house maintenance tip) and if its good/interesting enough, people will share it with their friends, they'll see it, see your page and like it/use your services.
We all do it, if we see something interesting or really funny, we tag our friends in it, we share it.
Hope this helps.
Meridion