Boy, is that ever an understatement. Wyclef pretty much had it when he said "There are at least as many versions of Socialism as there are of Capitalism." Communist Manifesto era terminology, where Marxism, Communism, and Socialism were one and the same, is pretty much irrelevant now, what with the advent of Marxism-Leninism, the more recent need for the left to dissassociate itself from Stalinism, and so on. But, at least in Europe (this does not apply to the US where the word still enjoys cussword status), definitions are beginning to consolidate and socialism nowadays means little more than state ownership of just the major industries, and Socialist parties subscribe to this doctrine and occupy a halfway libertarian stance on (most) social issues. Basically the equivalent of the far left of the Democratic Party in the US.

Marxism has become synonymous with relatively moderate Marxism-Leninism (oxymoronic but true, in most countries they're not legally allowed to advocate revolution), which has also become more or less synonymous with Communism, because that's the stance most European Communist parties hold. (Note -- there are generally rifts in the Communist parties between the pure Marxists, the Marxist-Leninists, the in-betweens, the whatevers, etc., etc.) And the two segments of the left pretty much hate each other as much as (or more than!) than they hate the right: Communists/Marxists think Socialists have sold out, and Socialists think Communists give the left a bad/radical image.

So no, Socialists are not necessarily Marxist - infact, these days they almost never are.