My great aunt, newly married at age 18, emigrated from Belagorodka to St Louis before 1900.  She had many relatives from Zaslav [and Shepetovka] some of whom joined her in the early years.  Her husband [Icheal/Israel Rich] had kin who were in St Louis.  I suspect that as has been mentioned, St Louis was seen as a hub of opportunity and discussed well in advance of immigration. 
Shirley Ginzburg

RICH [St Louis], BOCKSER & SCHLAGER/ SHLUGER [Boston, Lynn MA]  all from Zaslav & Shepetovka area, Ukraine
KANTER/OWITZ & dEMATOFF [Mir, Belarus to Chicago]