Actually, it's quite clear -- all organizations must go through a screening process, and the process is detailed on the site.

Indeed, it is not a widely publicized service. However, as with all volunteer matching services, there are always far more people who want to volunteer than there are opportunities. I just posted to the site last month, and as usual, had to take most of my opportunities down after just 24 hours because so many people applied.

What's needed is NOT more advertising to potential volunteers -- there are plenty of those. What's needed is more advertising to potential organizations.

And, yes, the UN has many organizations with whom it collaborates, but you seem to have the notion that the UN is one big organization -- it's not. The UN is made up of several different organizations, such as UNICEF, UNDP, UNIFEM, UNESCO, UNV, etc. And each of these UN branches have their own relationships with various NGOs.