If you're looking for snack food in St. Petersburg, you'll have no trouble finding the standard range of international burger and pizza chains. If you want something with a bit more flavour and local character, however, there is a small but increasing number of homegrown offerings worth exploring.
The arrival of McDonalds in Russia in the early '90s was a seminal indicator of the changes the country was undergoing, as locals queued for hours no longer for bread but for burgers and fries. Other international chains such as Pizza Hut and KFC followed in time, but with decreasing levels of popular enthusiasm. Attempts at creating homegrown equivalents were initially disastrous (the grey burgers and soggy fries of a certain "Burger Kvin" in Moscow became a standing joke amongst expats). The first truly successful Russian fast-food venture was Teremok, which began as a chain of street kiosks serving bliny (Russian pancakes) and kvas (a soft-drink made from fermented rye bread) and has expanded to offer dozens of restaurants in St. Petersburg - as well becoming an ubiquitous feature of shopping mall food courts - with an expanded menu that includes a variety of "traditional" dishes in fast-food format.
The other traditional staple that has been successfully translated into a modern fast-food offering is the Russian pie (pirog when full-size and served by the slice, pirozhok when offered as a single-serving bun). The most common fillings include potato, cabbage, mushrooms, chicken and salmon, and good pirozhki are filling, cheap and very portable, making them ideal for long train journeys, days out sightseeing, or as a respectable offering if you're invited to somebody's home for tea. A reasonable selection can now be bought in most supermarkets, as well as convenience stores and cafes like the Stolle chain.
The only other "local" fast-food widely available in St. Petersburg, especially outside the city centre, is shawerma (shaurma, doner kebab), but as the Russian version doesn't even have chili sauce to disguise the dubious provenance of the other ingredients, it's probably best avoided...