I can’t seem to find the stat, but I don’t think it’s anywhere near 10-15% if my memory from visiting serves me right. I believe it it closer to 2%. I am a current freshman and everyone I know is planning to do at least one co-op. Only one person I know is planning to do a single co-op, the rest are planning to do two or three. I myself am planning to do two co-ops.
As far as the social part of it. You wouldn’t even notice someone else isn’t doing co-ops until three or four semesters into your academics excluding directly talking to them about it. I don’t think you would be an outcast. If anything, it makes being social easier because you would be on campus more.
There’s plenty of other great things about Northeastern. For me, it being a city school was big. The size also means that you have plenty of resources no matter what you do. The CS program stood out, as do many at NEU (Business, 6 Year PharmD Program, Engineering) and all of the academics really do hold their own. The co-op was really about 1/4th of my decision, split with the rest of that. I am in love with the school and co-op isn’t the mean reason. I won’t even really experience it for another semester at least.