The Bolshevik coup in 1917 pretty much ended democratic socialism in Russia. Prior to that, workers were forming self-organized soviets (essentially a union government) that, at least from my vantage point, looked pretty much exactly like what a communist government is supposed to look like. Lenin didn't think Russia was ready for socialism yet, so the Bolsheviks seized power away from the soviets and formed a dictatorship. I'm not a socialist (well, I probably am according to today's definition), but if I were I think I'd say that's when the Soviet Union stopped being an effective socialist state.

Edit: just to clarify, this is a brief gloss that leaves out a lot of nuance.