“That's a fair point, but it can be a bit like the chicken and the egg, especially for more lifestyle type businesses looking to expand.
But even with free advertising/promotion there are still costs, ie your time (unless you work for free!)
I'd imagine your line of work would be quite receptive to local offline networking groups, or at least 1 or 2 meetings then move on to the next. No point peddling to the same people 
If you want to capitalise on xmas cards, then look at bulk. Can/have you approached businesses? Lots of bigger businesses send out supplier cards etc. Can you do designs incorporating commercial logos? Maybe they would be receptive to the charity angle? Big business is very sensitive to be perceived as being a good corporate citizen these days.
Lots of shoe leather, door knocking and making contacts will be the way forward. Waiting for a huge surge from social media will leave you still sitting there next xmas
”
Approaching big businesses is exactly what I was thinking. I was also thinking do Estate Agents send a Christmas card to a customer spending their first Christmas in a house they have bought from the Estate Agent?
How best to approach businesses, e-mail, phone call, post, in person?
Daunting but feel I will have to come out my comfort zone if I want to get ahead of the game.