The biggest in the region, Kuopio-Siilinjärvi, has an excellent runway 2800 m long and some population nearby, but it's located already over 400 km from St. Petersburg.
I was in Helsinki recently and there are train AND bus services from Helsinki to St Petersburg.
Coaches (buses)
So I don't imagine a 400km journey would be too much for people to cope with if they really want to travel.
it fits with the FR mantra - "if it can be in the middle of nowhere and costs next to nothing we'll fly there."
look at beauvais, charlerois, hahn, torp, weeze etc etc etc.
I'm not slagging off FR. In fact I'm saying they've been able to fill the seats and make it work from what many - including me - would consider obscure places. If it works it works. It's making some people happy.