What is the priority to be in the membership of the Hostelling International ?

Well, you are in Hostelbookers, so you should get some useful results there. I see that you have a pretty high rating.

And in the process of looking, I saw their new site, it looks really good to me. Speaking as an affiliate, it gives customers the same info if they have come through UKTrail as if they had come direct. Hostelworld, by contrast, provides customers coming through me with just the bare info, no ratings etc. At least, they did when I last looked.