The difference that arises at the core idea of fascism and socialism makes them two whole different ideologies. However, if you put that fact aside, you will see that both fascism and socialism are ideologies where strict rules are applied to the members of the society.
In his authoritative definition of Fascism (he coined the term,) Mussolini rejects classical liberalism because it makes the individual primary instead of the collective (specifically, the state--fascism is state socialism mixed with a limited free market.) Yes, he rejects Marxism and the Marxian doctrine of historic materialism. And yes, he rejects pure democracy, and argues for a class-based society. But he argues for "authoritarian democracy," where those who rule are expected to do so based on the greater good for the greater number (sound familiar?) Just like modern state socialists, he argues for the necessity of having the state regulate the individual and the market for socialist goals and objectives, using socialist justifications:
"Fascism is therefore opposed to all individualistic abstractions based on eighteenth century materialism; ...""Anti-individualistic, the Fascist conception of life stresses the importance of the State and accepts the individual only in so far as his interests coincide with those of the State, which stands for the conscience and the universal, will of man as a historic entity (11). It is opposed to classical liberalism which arose as a reaction to absolutism and exhausted its historical function when the State became the expression of the conscience and will of the people. Liberalism denied the State in the name of the individual; Fascism reasserts the rights of the State as expressing the real essence of the individual (12). And if liberty is to be the attribute of living men and not of abstract dummies invented by individualistic liberalism, then Fascism stands for liberty, and for the only liberty worth having, the liberty of the State and of the individual within the State (13). The Fascist conception of the State is all embracing; outside of it no human or spiritual values can exist, much less have value.
No individuals or groups (political parties, cultural associations, economic unions, social classes) outside the State (15). Fascism is therefore opposed to Socialism to which unity within the State (which amalgamates classes into a single economic and ethical reality) is unknown, and which sees in history nothing but the class struggle. Fascism is likewise opposed to trade unionism as a class weapon. But when brought within the orbit of the State, Fascism recognizes the real needs which gave rise to socialism and trade unionism, giving them due weight in the guild or corporative system in which divergent interests are coordinated and harmonized in the unity of the State (16)."
That's essentially modern China. And essentially the way most nations are actually governed, regardless of their marketing propaganda. Mussolini, at least, was honest.
Reference: http://www.worldfuturefund.org/wffmaster/Reading/Germany/mussolini.htm
Socialism is the idea that other people owe you something for nothing.
Democratic socialism is the idea you owe society something because you live in it--even though any such debt, to be valid, would have to apply equally in both directions, leaving a net debt of zero.
'Libertarian socialism,' a.k.a., anarcho-socialism, is the idea that other people owe you something for nothing and somehow it will all be distributed without violent overseers. History shows that to absurd:
"When under the pretext of fraternity, the legal code imposes mutual sacrifices on the citizens, human nature is not thereby abrogated. Everyone will then direct his efforts toward contributing little to, and taking much from, the common fund of sacrifices. Now, is it the most unfortunate who gains from this struggle? Certainly not, but rather the most influential and calculating." ~ Frédéric Bastiat
Iron law of oligarchy: "sociological thesis according to which all organizations, including those committed to democratic ideals and practices, will inevitably succumb to rule by an elite few (an oligarchy). The iron law of oligarchy contends that organizational democracy is an oxymoron. Although elite control makes internal democracy unsustainable, it is also said to shape the long-term development of all organizations—including the rhetorically most radical—in a conservative direction.
Robert Michels spelled out the iron law of oligarchy in the first decade of the 20th century in Political Parties, a brilliant comparative study of European socialist parties that drew extensively on his own experiences in the German Socialist Party. Influenced by Max Weber’s analysis of bureaucracy as well as by Vilfredo Pareto’s and Gaetano Mosca’s theories of elite rule, Michels argued that organizational oligarchy resulted, most fundamentally, from the imperatives of modern organization: competent leadership, centralized authority, and the division of tasks within a professional bureaucracy. These organizational imperatives necessarily gave rise to a caste of leaders whose superior knowledge, skills, and status, when combined with their hierarchical control of key organizational resources such as internal communication and training, would allow them to dominate the broader membership and to domesticate dissenting groups. Michels supplemented this institutional analysis of internal power consolidation with psychological arguments drawn from Gustave Le Bon’s crowd theory. From this perspective, Michels particularly emphasized the idea that elite domination also flowed from the way rank-and-file members craved guidance by and worshipped their leaders. Michels insisted that the chasm separating elite leaders from rank-and-file members would also steer organizations toward strategic moderation, as key organizational decisions would ultimately be taken more in accordance with leaders’ self-serving priorities of organizational survival and stability than with members’ preferences and demands." ~ Encyclopedia Britannica