From my experience in Europe: hostels imply a bunk bed in shared room here too. I just made a quick search on booking.com for a hostel in Warsaw and nearly all of the offers were shared dormitories.
I must have stayed in something like 30 different hostels in Poland over the course of last decade; it was always in single bed or two-bed rooms, with the whole room rented for me. I never used booking.com, though, but always did a Google search for "hostele w $city", and reserved room via phone or web form.