It’s often said that after the Winter War, the inhabitants of Finnish Karelia were offered Soviet citizenship and the right to stay in Karelia, and that almost all left.
Given the not inconsiderable proportion of Finns who were communist sympathizers,(The communist-dominated party won 23 percent of the vote in 1945) and the natural attachment to home people, and in particular peasants, have, I rather doubt they all left voluntarily.
Alexander Turok 
Finnish communists were mostly not communist “sympathisers” keen to be ruled by Russia/USSR, although that would have been their fate.
The attraction of communism as a political system was to give more land and rights to working people, given that Finland in living memories had freed itself from Swedish then Russian sovereignty, only in 1919.
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