You've all got it ass-backwards.
For one, Proudhon and Marx are not nearly as opposed as you think, as I will illustrate later, as it is central to a much greater point.
Second, Proudhon was not a very good political theorist, and as a whole, his philosophy leaves much to be desired. One of Marx's early monographs,
The Poverty of Philosophy, was written to criticize the hopeless utopianism in Proudhon's work
The Philosophy of Poverty. More importantly, Marx wrote this while he was living in France, and it made considerable stirs among French socialists. Although, admittedly, it's popularity may have something to do with Proudhon being, in person, not a very nice guy and a sexist, racist dick. So a "Proudhonism" isn't really possible, since Proudhon really couldn't be much of a one-man show. Which isn't to say that his
mutualisme couldn't take on a greater level of popularity.
And here's the kicker: in spite of the enduring legacy that Marx has had as a philosopher within socialism/communism, Marx didn't win the ideological battle for control of the First International. He, along with Bakunin and the followers of Proudhon, were very handily beaten by German liberal democrat turned social democrat Ferdinand Lasalle and those who shared his very middle class professional, Victorian sensibilities.
The people who would come to dominant the socialist movement in the latter years of Marx's life, and for the decades following may have paid their homages to Marx, but by any reckoning, they were very poor "Marxists". Marx's later writing was almost entirely devoted towards organizing against the statist, highly vulgar type of social democracy that was becoming prevalent in European socialist parties. And Engels unfortunately contributed to this, by both being a poorer intellectual than Marx, and by suppressing the publication of much of Marx's more critical work towards the main institutions of the socialist movement, the German Social Democrats in particular.
Marx's influence in the European socialist movement was always very superficial. Marx was a convenient place to draw quick inspiration from for men like Lasalle--very much bourgeois and classically educated men whose socialism was a very thinly disguised nationalism--because he was a brilliant polymath and very meticulous scholar. His works could give a great intellectual over-awing to the uninitiated. And since Marx writes with the tone of a social scientist in most cases, and not a polemicist, it is quite easy to borrow him to support a wide variety of positions and politics.
But Marx stood very strongly opposed to the kind of politics that the social democrats had arrived at. For example, in the Gotha Programme of the German Social Democrats, they placed an incredible emphasis on, as Marx pointed out, a mistaken belief that there was an iron law of wages in capitalism that kept everyone poor, and that under the wage system, man was being horribly exploited. But as Marx wrote, this sort of criticism wasn't just based on mistaken assumptions about the nature of present society, but also fundamentally ridiculous. "It is as if, among slaves who have at last got behind the secret of slavery and broken out in rebellion, a slave still in thrall to obsolete notions were to inscribe on the program of the rebellion: Slavery must be abolished because the feeding of slaves in the system of slavery cannot exceed a certain low maximum!"(
Critique of the Gotha Programme, in
Marx/Engels Collected Works, Vol. XXIV, pp. 92.)
He also took them to task for their statism. As he wrote:
First of all, according to II, the German Workers' party strives for "the free state".
Free state — what is this?
It is by no means the aim of the workers, who have got rid of the narrow mentality of humble subjects, to set the state free. In the German Empire, the "state" is almost as "free" as in Russia. Freedom consists in converting the state from an organ superimposed upon society into one completely subordinate to it; and today, too, the forms of state are more free or less free to the extent that they restrict the "freedom of the state". (Ibid., pp. 93)
I apologize if I sound catty about this, but these are misconceptions that are nigh universal, even among the very well educated. I can only think that this is a result of the very little emphasis that education, even higher education, puts on studying political philosophy. It's not as though Marx is a dense writer or is someone who writes in an intentionally obfuscating manner either: there is simply no impetus to give students even a cursory education in the rigors of politics and philosophy, let alone give them the tools for further self-study. Thus, misconceptions and myths persist, and Marx is particularly vulnerable to this because he was a favorite of Lenin, who may have been a better "Marxist" then Bernstein, Lasalle or Kautsky, but for everything he got right in theory he got two wrong in practice, and obfuscated all his authoritarian actions in Marxist terms. Thus Marxism-Leninism is born, and Marx is trotted out to support every "Communist" single party state, serving as the personification of the official ideology of the state supporting the rule of the
nomenklatura over the proleteriat.