This was an interesting read My memories are rather old, but:
Your timing was when all students study very very hard. It was about the week when they have to take tests and submit papers for the courses graded on pass/fail basis. Since 95% of students do not do much during most the semester, they try to make up for this in the end of December.
For St. Petersburg, 12 am must be way too late. The city has a big center, broad old industrial belt, and only then "sleeping districts". When metro closes, how younger people would go home? Night buses, even if available, must be slow and can not cover much of the "sleeping districts". Taxis are too expensive for students.
As for the clubs in St. Petersburg, maybe Bunker would provide more opportunities: http://bunkerspb.ru/
Besides bluesystem , there is also qguys