Socialism fails sooner or later. The key factor is the degree an economy is infected with socialism. A moderate amount of socialism (that is anti-price government intervention) means a moderate collapse eventually. Total socialism means total collapse of the economy eventually.
There are 2 very important variables in this simple equation however (there are of course more than 2, but these are the biggies) and that is how much fear a regime can induce in the population, and how far a regime is willing to go to stay in power.