[Paradigm]

I know Spooner at one point was a member of First International, but that doesn't make him a socialist (the aim of the organization, IIRC, was to unite various leftist/socialist/anarchist groups). Today he's identified most closely with libertarianism/anarchy. In fact your post is the first time I've ever heard him referred to as "socialist".

He wasn't too fond of wage labor either:

“...almost all fortunes are made out of the capital and labour of other men than those who realize them. Indeed, large fortunes could rarely be made at all by one individual, except by his sponging capital and labor from others.”

— Poverty: Its illegal cases and legal cure.

[Technology]

As late as the early 20th Century, Benjamin Tucker was sometimes considered, and called himself, a socialist (though the distinction between social and individualist anarchists was still made) for supporting the labor theory of value.

As Marxism ascended, its more exclusive definition of socialism took over... but only amongst politically educated people.

By the 21st Century, Marxism had become a byword for dictatorship amongst the common person in the West, and among Teaheads socialism now means someone putting in place social programs and restricting corporate activity. In response to this, more and more left-liberals are starting to wear the label "socialist" proudly, while still advocating distinctly liberal-capitalist policy.

[Paradigm]

As late as the early 20th Century, Benjamin Tucker was sometimes considered, and called himself, a socialist (though the distinction between social and individualist anarchists was still made) for supporting the labor theory of value.

One litmus test I would make for socialism would be opposition to wage labor, which is true of both Tucker and Spooner. There is more to it, of course, and Spooner's views on property were more in line with the ancaps who currently revere him, but I would still say that his views on wage labor and support for labor struggles puts him at least marginally within the sphere of socialism. Socialism essentially advocates replacing an economy driven by profit with one based on social need. It is not necessarily incompatible with markets, so long as money only serves as a means of exchange (C -> M -> C) rather than a source of accumulation (M -> C -> M').

As Marxism ascended, its more exclusive definition of socialism took over... but only amongst politically educated people.

Anarchism has always presented another vision of socialism alongside that of Marxism. I would say that Stalin corrupted its definition more than anyone. Lenin recognized that the system in the Soviet Union was an attempt to create socialism, but was not itself socialist. Stalin made the revisionist claim that the Soviet Union had already achieved socialism, even though it was confined to one country and still had wage labor (albeit with the state taking the role of the capitalist). And while Stalin was claiming the legacy of socialism, the US was denouncing the Soviet Union and using it to smear socialism by associating it with Stalin the authoritarian system he oversaw. So when you have the two great superpowers both defining socialism the same way but for different reasons, it drowned out the voices of the other socialists -- both anarchists and other Marxist tendencies -- who had been there all along. On the other hand, European-style social democracy also came to be referred to as socialism, which has also led to some very confused definitions. Social democracy and Stalinism both had the feature of having a large role for the state in the economy, so many people came to associate socialism with "big government," which is why you get all these Tea Party folks claiming that Obama is a socialist for passing a healthcare bill and liberals complaining about bailouts as "socialism for the rich."