I remember being surprised when I began doing some of my Slovak ancestry reading, after the 1990 U.S. Census, the fact that Slovak was the second highest ethnicity for Slavic people in the U.S. Although the numbers are hard to pin down, about a half million Slovaks immigrated to the U.S. during the 1880-1920 time period. However, with the end of the Soviet Union, Slovaks are now falling behind other more recent Slavic ethnicities, like Ukrainians.
The other interesting fact I found surprising was that as a percentage of those that left "the old county" the Slovak immigration to the U.S. is second only to the Irish exodus of the 19th century. That's partly due to the fact there were only about 4-5 million Slovaks to begin with.