Now many people have an account on various social networks. If you only know the surname and the name of the person and at least his approximate age, you can start your search there. In order to begin the search in social networks such as "Classmates", "Vkontakte" or "Facebook", you enter the name of the person and the city of St. Petersburg. If the person you're looking for, is there a page with real data, you will find it.
If the name is common, and a couple of people with such data, see pictures (of course, if you know the person in person). Of course, the more you know about the person, the more likely that you will find it. Surname, name, age, years of study, University, school or work will help to bring you closer to your goal.
If you a person is not found, try to look otherwise. There is a project like http://wikilife.ru. There you can also look for a person on the minimum data. For this you do not even need to be registered in the system. Simply enter in the form to search for what I know (quite a name, surname and year of birth). The project is free.
Another way is telephone base of St. Petersburg residents, its address http://www.nomer.org/spb/ . In order to find a person, enter his data (name, surname, patronymic). The website provides information for residents of St. Petersburg and the nearest suburbs, it is almost three and a half million records.
Try typing in the address bar of any search engine the name of the person and the city of St. Petersburg in different variations. Sometimes you can find the information.
If an independent search on the Internet failed, you can go to the Address Bureau of the police Department of St. Petersburg and Leningrad region. It is located at the address: St.-Petersburg, the Foundry prospectus, the house 6. There must come in person on a weekday during business hours. You must fill out a form where to specify all known data about who you're looking for, and pay receipt. The response you get after a while, maybe in a few days.