Socialism has not, currently does not, and never will work. Regardless of your particular type of "socialism" (eg. Marxist, Leninist, State Socialism, etc.), the key point is that it is going to take the means of production out of the hands of the "capitalists" or "landowners", and distribute it evenly to either the works or the state or some other similar combination. In theory, it sounds nice - why shouldn't the workers have full control over the labour? But such a simple analysis leaves much left to be desired, and leaves out crucial driving factors that are essential to the human race such as incentives, greed, hierarchy of needs, etc.
Looking strictly at Marx, and in particular, his work Capital, he theorized that the falling rate of profits in the capitalistic society around him and rising rates of exploitation would continually cause class tension, eventually leading to a revolution. From there, he hoped that a newly formed state would arise to the benefit of workers, but he didn't write much about what that sort of government should look like.
In general, Marx said that the value of a good was a function of the following 3 things:
Labour used to produce the good (variable capital)
Machinery used to produce the good (constant capital)
Surplus value (whatever is left over after the good sells and the monies are used to pay labour and machinery)
Marx therefore tried to prove that if a commodity sold for a price above the cost of paying labor and paying for machinery (i.e. depreciation), then exploitation over the workers was occurring since the owner of the means of production was pocketing the surplus value from the good being sold. Further, this would cause incentive among those who owned the means of production (i.e. capitalists) to try and keep the cost of labour as low as possible by ensuring that the supply of labour was large relative to demand (reserve army of the unemployed) and to advance mechanization by making machinery cheaper and more efficient.
One of the main issues is that Marx's analysis (arguably the foundation of socialism) relied on the assumption that the rate of time preference (i.e. the interest rate) was zero. Put more simply, Marx assumed that when a capitalist invested in machinery, so that the capitalist could combine his machinery with labour to produce a good, there was no opportunity cost for him, and therefore the capitalist did not deserve anything more than money to pay his labor and replace his machinery. However, this does not reflect reality at all. Interest rates exist, and act as an incentive to either invest in machinery or invest in something else, depending on market conditions. Why would a capitalist invest in machinery to produce a good (when combined with labour) if he could instead place his money in a savings account or bond market and enjoy a return of X%? Marx's assumption that there was no rate of time preference (opportunity cost of investing in machinery and expecting a return) was a critical flaw.
As humans, we operate based on incentives. There are incentives for you to get a better education because you think that your long term earnings are going to exceed the cost of going to school. There are incentives for you to get a job because you believe that you selling your labour is going to be more worth your while when compared to whatever else you would do if you didn't sell your labour.
Similarly, people with money use incentives to put their money to work for them, which ends up benefitting many other people. If the current interest rate on a savings account is 1% and someone has $100,000 to play around with, they may find that starting up a small restaurant may net a return of 3% per year. Therefore they go out and buy a building and pizza oven, hire 10 employees, and begin producing food. At the end of the year, they may have invested $100,000, but the return is $3,000 and their business is still worth $100,000. For the employees, instead of getting the $3,000, they get whatever wages were promised to them in their initial contract with the capitalist.