The question raised was can someone fulfill his obligation of Purim to donate money to the poor, matanot la'evyonim, by donating via the major, central, charities such as Kupat Ha'Ir and Vaad HaRabbonim, among others.
ou cannot
Rav Ephraim Greenblatt wants to say that you cannot be yotzei the obligation by donating through these large organizations. He bases his opinion on an opinion that says the donor and the poor person should know each other, as they normally do with regular gifts. When the poor person does not know from whom he received the gift, and the donor does not know to whom he gave it - the opinion of Rav Yosef Engel is that this is not conisdered matanot la'evyonim.

Rav Greenblatt explains that the intention of matanot la'evyonim is to bring the two sides closer to each other, to increase love and friendship. According to Rav Yosef Engel, he says, this does not happen when they do not know each other.

He agrees that nowadays we do not pasken like this, but there are those who suggest that one should attempt to fulfill the mitzva even according to this opinion, by giving to a poor person you know rather than through an organization.

Rav Greenblatt presented his opinion to Rav Wosner and to other poskim.

Among the poskim, some say that there are additional reasons to not give through the large organizations. They explain that when the pot of money is divvied up and distributed among all the numerous recipients, there is a good chance that from your donation you will not have given a shaveh pruta, the minimum amount, to any one person.
While perhaps this is not the final psak of how the money gets divvied up and whether or not you are yotzei, some of the rabbonim do say that to fulfill the mitzva the best possible way would be to give the donation directly and not through the central kupot tzedaka. (source: Bechadrei)

It sounds like it should not be specific to the large kupot, but even to neighborhood kupot,. The same logic seems to apply.

Practically, it does not seem practical for most people to go search out for a poor person they know, as most people will not, or do not know any halachically poor people, or more precisely perhaps, do not know that the person they know is halachically a poor person. The kupot provide the added benefit that they ensure your money is being given to people who are genuinely needy and halachically qualify.

I am sure we have not heard the last word on this topic...