Salam Alaykum, 
if a man met a woman online through marriage or matrimonial platforms ( or social media with the intention of marriage) , is this considered haram ? And can a parent/wali refuse to have the man approach to ask for marriage to his daughter? Due to cultural or societal norms. 

assuming the man and woman both have halal intentions of marriage and both agree to seek permission from wali. Based on the statement from Seyed Sistani is a father allowed to refuse to meet with the man interested ( not necessarily guarantee to marry)
 
from Seyed Al Sistani : 
Guardians are not permitted to prevent their daughters from getting married, nor are they permitted to put obstacles in their way using improper traditions not required by God, such as asking for exorbitant dowers or waiting for cousins and sayyids, because there are many sources of corruption in these traditions. God made fathers the custodians of their daughters only so that they counsel their daughters and choose the best option for them. One who confines a woman for other than her interest has committed a sin that endures as long as her suffering, and has opened one of Hell’s doors upon himself.”
Full Article: http://www.sistani.org/english/archive/25240/