Unfortunately I'm unfamiliar with Social Science, but I'm fairly certain it depends on the specific program you're trying to get into? (ie. Economics, Anthropology, etc) You can likely find this information on the Social Sciences webpage, here:
http://www.socsci.mcmaster.c a/Graduate/graduate.html
But I do know that if you don't reach this requirement you're not kicked out. You're put into a 'general program' which allows you to take the same courses and 'shadow' the program you're hoping to get into. ie. if you fail to get into Anthropology, you can take 2nd year Anthro courses in your 2nd year.
Then if you 'show improvement' in your 2nd year you'll be admitted in your third year. (meaning, raise your average to the required average, etc).
Good luck!