Now to answer the question:
If we want to look at specifically the USSR (as West Germany kinds disproves this argument) then Marx accurately predicted an inherent problem with his socialist model.
Karl Marx understand benefits of capitalism, he acknowledged that capitalism is great at creating wealth, but his problem with it was that this new wealth would be concentrated into fewer and fewer of the bourgeoisie.
He therefore advocated a capitalist society that would build up the industrial base required for socialism and then have a proletariat takeover of said industry.
He specifically referenced Prussia, France and Great Britain as the areas that the socialist movement would take off, simply because Prussia, France and Great Britain where the only countries with the industrial capacity that he thought met the needs for socialism to work.
Russia in 1917 was not ready for socialism, and Stalins 5 year plan was required to change that, but his 5 year plan fell short, and despite its rather successful implementation (excluding the millions of people who died as a result), the USSR was still unable to compete with the far superior industries of Western Europe and USA.
This was the reason for the inevitable downfall of the USSR, the country simply lacked the resources, both land (using the economic, not geographic definition of land) and capital to implement socialism properly and as a result couldn't compete with the US. It wasn't a failure of socialism that caused the downfall of the soviet union, but the constraints left on the poor country by the people, both externally and internally expecting to much of the young socialist republic.
Karl Marx wasn't expecting socialism to compete with capitalism, but to replace it and in this sense Karl accurately predicted things to come. Europe (and now America following it with the election of Obama) is becoming more left wing and the benefits of Marxist economics are starting to be realized in Europe. People now have access to free healthcare and other goods that are expecting in a socialist society but absent from a capitalist one.
The revolution wasn't a literal revolution, but the proletariat using his power in the voting booth to slowly implement socialism in the way that Karl Marx envisioned.
you can see the socialist leaders who have won numerous elections across Europe as proof that this has happened.
This is the argument that I would use, from a socialist perspective, of why the USSR failed.
Thought I am sure Putin is going to come in and start blabbering that the USSR failed because Gorbachev wanted it to and it would have won the cold war if a more Stalin like leader was in power during Gorbachev's time.