For many years I tried to arrange a network of home stays or bed and breakfasts in St.Petersburg, but in the end it turned out to be impossible for me.
As to the reason, well let us look what makes modern Russia different from old good Europe - there is almost no middle class in our country. There are people who are very rich, live in proper palaces and have neither need nor desire to host any strangers. There are also open, nice and generous Russians who would love to host somebody from a different country, to share what they can about Russian culture etc. But such people usually live in tiny, and in the best case, very modest places. Realistically they can not offer even half way decent conditions for a guest. That is why the market practically does not exist.
So if you looking for a home stay in St. Petersburg my best advice would be: don't waste your time, it is not a place for that. If you want to find friends among locals - just to chat about life, then try to search the forums of English language schools. People who study English are usually keen to meet and chat. As for accommodation look into mini-hotels for central, but modestly price accommodation. I can help you with an advice with this based on personal visits to such places. Contact me with your dates and requirements I will try to recommend something good.