What opinion/ message/ lesson is Orwell attempting to convey to the readers on the subject of "socialism" in Animal Farm?

Animal Farm is an allegory about what went wrong in the USSR, which initially adopted socialism as an idealistic, utopic experiment meant to liberate workers from exploitation and share the fruits of labor equally, but which quickly turned into a totalitarian regime that robbed people of their rights and their labor. Orwell, writing at a time when many people still defended the Soviet Union, meant to show that a well-intentioned revolution can easily derail. Key to this is language: the pigs are easily able to control the rest of the animals because they control the rhetoric. Language is very important, Orwell argues. We have to maintain vigilance over what is said and how it is said. The animals end up deceived because the pigs continually change the rules of the game through word games, finally changing "all animals are created equal" by adding to it the famous non-sequitur that some are more equal than others.
Violence--or putting controls on it-- is also important to maintaining freedom. Orwell illustrates this by showing how the pigs maintain a monopoly of violence by controlling the dogs, who act as executioners against the other animals. This combination of rhetoric and violence keeps the rest of the animals down, which is, of course, what happened under Soviet socialism--and more to the point, could also happen in nominally democratic states like Orwell's home county, England--or in the United States. 

Orwell's novella is an analogy for the events leading up to the Russian Revolution and Stalin's eventual rise to power as leader of the corrupt Soviet Union. Initially, Orwell praises old Major's beliefs of equality and camaraderie among the animals. Old Major represents Karl Marx and Vladimir Lenin's socialist beliefs. Orwell believed socialism was the most egalitarian form of government and supported its ideologies when compared to capitalism. Unfortunately, the pigs consolidated control over the farm and Napoleon usurps power and becomes a tyrannical leader. Orwell also believed all people are capable of becoming corrupt, which is why some forms of socialism can be dangerous. In many socialist governments, power is consolidated, and decisions are made by a select few. Animal Farm is essentially a warning to citizens that a socialist form of government can become corrupt and easily morph into an authoritative regime.