I agree that for most of China's history between 1949 to present, "Maoist" policies such as the Great Leap Forward and the Cultural Revolution generally resulted in economic crises. However, that is not an argument against socialism. During 1950 - 1958, it is widely recognized that China managed to recover its economy and achieve high growth rates using the Soviet model of socialism. Similarly, the Soviet Union during the late 1920s to 1930s achieved high rates of growth, albeit under great repression.
What destroyed both the Soviet Union and China was a failing in political, not economic, thinking. Furthermore, they failed to reform their economies, such as the way Roosevelt reformed the economy after the Great Depression. In the Soviet Union's case, they continued the Stalinist model after the destruction of World War II, and failed to raise their standards of living as a result. In China, Maoist political factional struggles was what undermined the high growths of the socialist economy. (You'll realize that between the gaps 1949-1958, 1962-1965, when there wasn't massive social upheavals, the economy grew quickly.)
Also, socialism isn't just North Korea and Cuba. North Korea, by the way, had a great economy (much better than the South) until the 1960s or 1970s (I forgot), when North Korean leaders started to develop some crackpot shit economic theory that places the military first. Socialism is most simply a progressive tax system, with greater benefits to the people (health care, education), and less income inequality, etc. This does not mean equal wealth for all; a person who doesn't work hard, for example, will get basic education and health care for free, but won't enjoy a lot of money to buy consumable goods. Hard-workers can enjoy some extra profit to enjoy some consumable goods.
Capitalism is not "working". To say capitalism is a good system plainly based on the world's current GDP growth is insane. Slave economies historically have grown equally fast; does that mean we should all start using slaves? Stalinist Russia grew at massive rates during the 1930s; does that mean purging and repression is the way to go?
Whatever though, keep on believing that we can steal money from people and violate basic economic principles - when you actually have to make your own money, or at least understand the basic workings of making your own money, you might see things differently.
Capitalism, admittedly, is not as simple as 'exploiting' money from poorer people, as many lesser socialists might argue. However, it is undeniable that a degree of imperialism is attached to capitalism. The U.S.A is a great example.(what with South and Central America, exploits in the Middle East, Southeast Asia,and I'm probably forgetting something)