Of course, you don't need to trust completely to people you just have met, drink with them and invite them to your accommodation or go to their house. About money and taxis safety tips I have already written above. In general, be careful like in every trip. Communication in St. Petersburg. Here I explain possible questions about telephone system, codes and communication in St. Petersburg.
Many people think that it is difficult, but really it is easy.
The phone code of St. Petersburg is 812, Russia phone code is 7. Petersburg code and then the 7-digit number. If you call from other Russian city to St. Petersburg, you should dial 8, wait for a tone, then dial 812, and then the 7-digit phone number. If you're in St. Petersburg and call inside St.