Honestly, Marxist-Leninist socialism was implemented after the war in places that, while not the richest, were certainly advanced to fairly advanced industrial economies of the time, East Germany, Czechoslovakia, Hungary, and Poland.
Guess what, they fell behind their contemporaries by 1990, and haven't caught up fully yet. Admittedly, eastern Germany was considered an agrarian backwater compared to the rest of Germany before WWII, so perhaps socialism can be considered just a chapter of that area's relative decline.
However, none of the above places were wealthy, despite being considered the best of countries of the Second World, and had a larger gap to make up economically in 1990 than they did in 1945.
While you're certainly right from a general perspective, your analysis should be corrected in some minor points. Let's consider East Germany, which wasn't an agrarian backwater (at least not Saxony), but which had been heavily damaged by WWII and the forced reparations to the Soviet Union. The history of the GDR wasn't a constant decline. East Germany was successfully rebuilt (albeit much slower than West Germany*) and re-industrialized under Ulbricht. During the 60s, the GDR exported many products that were known and appreciated even in capitalist countries. It was Honecker's policy of social welfare** that caused stagnation, indebtedness and finally the industrial decline of the 80s.
*Comparing eastern and western Germany is a bit unfair, since the West had some decisive advantages:
1) support by the Marshall Plan
2) a market economy (Ulbricht succesfully launched reforms to improve the planned economy, but these plans were cancelled after he was replaced by Honecker; these reforms, known as the NÖS or New Economic System angered the Soviets, and were one of the reasons for Ulbricht's toppling)
3) almost no war reparations (whereas the east had lost 45% of its economic potential and great part of its infrastructure to the USSR - for example, many railroad lines lost their second track)
4) most ressources, especially coal deposits, were located in the west
5) much less destruction of housing during the bombing campaigns
**The so-called Einheit von Wirtschafts- und Sozialpolitik (Unity of economic and social policy): Essentially the living standard of the eastern population was slightly raised, at the expense of budget balance and long-term investments.
Taking that into consideration, the GDR fared quite well, at least until it frantically tried to reach the west's living standards in the 70s and 80s - a policy that proved not only to be unsuccesful, but also destructive for the economy as a whole.