score:5
I don't think that there is a single popular platform for finding accommodation in Poland. There are at least few sites that seems to have decent amount of listings like nocowanie.pl or noclegowo.pl. Also Polish equivalent of eBay - olx.pl has some offers here.
Of course AirBnb is quite popular (especially in touristic places), but prices seems to be more on European level, so I guess targeted mainly to international tourists.
In rural areas (for example near the sea or mountains) there is a lot of people offering accommodation that are not listed on any search engines. They're advertised mainly by local signage or sometimes on their own website or some local site (like this local site that has just offers from area of ΕwiΔtokrzyski National Park). In cases like this decent way of finding accommodation is by recommendation - usually on local Facebook group where you will just get contact phone number. Places like this can be cheaper as they don't have to pay the middleman for advertis*m*nt (and I guess taxes).