beautiful architecture- it's much like mosques of Uzbekistan & Iran, great photo that combined architecture with nature & with people. maybe the mosque was under maintenance at that time & that's why they are praying outside, and why they are few probably because as you said everyone can pray inside the apartment/house, also because only 2.23% of st.petersburg population are muslims so probably if this was in Moscow there would be much more due to the much bigger muslim population made of both russian citizens & citizens of ex-soviet union, & of course if this was within Russia's muslim majority regions there would be even much more. have a good day!