When Locke heard Eko say "you're next" to him before dying, Eko's words meant:

John Locke is the next person that MIB will attempt to use as a "sucker" 61% (658 votes)

MIB is after all of the candidates and he will go after them all next 30% (328 votes)

Referring to Nikki and Paulo, as it was confirmed the medusas were the Monster 8% (90 votes)