someone who pokes randomly without intention of hooking up or making genuine contact with the pokee.
When you make contact with another person on facebook it is customary to actually follow the connection up with some sort of genuine human interest - hence poke slut - poking around
someone who has an absurd amount of pokes on facebook or any other social site
girl: i have like 50 fucking pokes on facebook
guy: your such a poke slut