Ok, I'm at York doing my MA and I have to actually agree that the social side isn't very open. I think it's something about it being a campus uni, it's quite insular sometimes. It's slightly different for me, I'm a part time post grad with kids - the GSA do lots of events but I never manage to get to anything around school, studies, hubby's shifts etc.
My advice, look outside the Uni. Try some other stuff that you're interested in. Join a gym class or a sports club. Go to WI meetings, anything that sparks your interest. Uni isn't the be-all and end-all. You will need to 'be' afterwards, so try to think about the afterwards bit rather than the now bit. Volunteer work? Trying to gain experience in your career field? Or even just working in a charity shop? Do you want to stay in York? Or will you go home/to another area? Lots of things to look forward to and focus on....don't be put off because you're struggling to make friends right now.