Find local groups and ask if you can advertise on their page - rugby clubs, football clubs etc
Also promote it to your customers by putting the Find us on facebook logo on your leaflets / your windows have been cleaned slips. You could do some kind of prize draw (free clean) or offer discounts through facebook (£5 off fascia cleans)
If your van is sign written, then stick the Find us on facebook on it
Im going to be doing the above on my next batch of leaflets & when i get me van sign written