There have been several waves of assimiliation only since the formation of the first Czechoslovak Republic in 1918 among the Rusyns who found themselves living within the boundaries of the Slovak portion of the republic (with the process pushed by the "independent" fascist Slovak state that existed 1939-1945 under Nazi control).
There's very little evidence that a measurable population of ethnic Slovaks pre-1918 were Greek Catholic (perhaps a village here or there that was reconverted from Protestantism in the Counter-Reformation chose the Greek Catholic Church over the Roman Catholic Church). That also accounts for ethnic Hungarian Greek Catholics -- as opposed to Magyarized Rusyns who live in modern-day Hungary.
Slovaks are a Western Slavic ethnicity and their liturgical heritage is Latin. Rusyns are an Eastern Slavic ethnicity and their liturgical heritage is Greek.
For Slovaks to adopt the Greek Catholic faith in the Austrian Empire or direct rule from Roman Catholic Hungary would have been a major step away from the cultural, political and theological mainstream and makes little sense, unless, as I said, there are cases of individual Protestant Slovak villages who adopted Greek Catholicism as a less reprehensible surrender to the power of Rome.
Many Rusyns identified as Slovak during Communist times because the state labeled Rusyns as Ukrainian, an identity that held little connection to the folks of Eastern Slovakia. Assimilation continues in Slovakia today (with the Greek Catholic hierarchy a major force behind it.
The confused ethnic situation in current-day Slovakia is verified by census data where tens of thousands of people who identify their ethnicity as "Slovak" say that Rusyn is their "mother tongue."
This sounds right according to what my dad has said growing up in the late 30s to 40s in Eastern Slovakia in the town of Gelnica. As he put it the only religious groups in his area were "Catholics" (latin), Lutherans, and Jews". Later on after the war and fleeing that land with the communists he noticed that he saw Ukrainian and other refugees that he suspects were East Orthodox. But the land he grew up in was largely populated by German settlers and the local western Slovak slavs. Although Hungary was a strong influence. My grandaparnets forgot their czech and slovak, but ended up speaking Hungarian and German while living in the states.
http://en.wikipedia.org/wiki/Gelnica
