Communism has not failed as an ideology or a movement. I would assert that there has been no legitimate Communist nation in the world since the death of Comrade Lenin, and that, had Comrade Lenin survived, Communism would not now be seen as a failed system.
Sadly, for the world, Comrade Lenin died before he had a chance to shape the Soviet Union into a truly Marxist-Leninist union. His death paved the way for the leadership of Comrade Stalin which, though successful in a number of ways, ultimately failed to produce the hoped for results. The moment Comrade Stalin died, the anti-Marxist-Leninist forces under the leadership of Krushchev were quickly able to denounce Comrade Stalin, and undo everything he attempted to accomplish. By the time Comrade Brezhnev came to power, it was too late to go back.
Mao Tse-tung may have been a Communist in his youth, but once he assumed power he quickly showed his true - imperialist - agenda. One of his first acts was the invasion of Tibet! His policies could not keep at bay the capitalist tendencies that had dominated China for centuries, and thus his most colossal mistake came into being: "The Great Proletarian Cultural Revolution". As Comrade Hoxha said:
"The course of events showed that the Great Proletarian Cultural Revolution was neither a revolution, nor great, nor cultural, and in particular, not in the least proletarian. It was a palace Putsch on an all-China scale for the liquidation of a handful of reactionaries who had seized power."
The abuses of the "Cultural Revolution" were foreseeable, and avoidable. Unleashing hordes of angry people into the streets to smash the "reactionaries" was a recipe for disaste. The violence and destruction, somewhat lessened by the heroic efforts of Comrade Zhou Enlai, could have been avoided had Mao been capable of containing the whirlwind that he unleashed (he either was not capable, or he did not care). The excesses of the "Cultural Revolution" led directly to the ascendency of Deng Xiaoping and his push to bring capitalism back to China. As a result, China today is as capitalist as the U.S. is.
Nothing need be said of leaders like Nicolae Ceausescu, Ramiz Alia, or Todor Zhivkov, except to say that they all hastened the end of Marxism-Leninism in their respective nations.
Mao's China was never truly Communist, as Mao was more concerned with the typical imperialist practices of the the day: conquest, power, and self-aggrandizement. The Soviet Union was well on its way to actually achieving a truly Socialist society, but was undone by those who were influenced by the power - and the wealth it brings.
Bob Avakian of the RCP makes outlandish claims to try and suggest that socialism did not fail. One such statement is that it was only a "first-stage" in an ongoing revolution. I should point out that capitalism has existed for centuries, and Communism in Europe died in less than 100 years. It also seems unlikely that the People's Republic of China will last to see its 100th birthday. If he is correct, it will be decades before Communism is likely to re-establish itself in any real way. If he's wrong, much work needs to be done to rehabilitate the image of Communism in the world. In either case, the road ahead is rocky for Communism, and smooth as glass for the capitalist exploiters.
Today, Communism in the United States is fragmented into dozens of small groups with no real power or influence. Communism in the rest of the world is beginning to follow the example of the CPUSA and integrate into the capitalist system. In those few areas where Socialism holds sway still, some, like North Korea, are brutal dictatorships, others, like Venezuela, are elected and could be voted out in future elections.
Communism needs to embrace its roots, to re-educate itself on the fundamentals of Marxism-Leninism. Until we can operate according to those fundamental principles, Communism will remain small, fragmented, isolated, and ineffective.