Postally speaking, the United Nations is an invention—an organization, not a nation or political entity. The fact that it issues its own postage stamps and maintains its own postal service is a concession to the trappings of sovereignty (real political entities control their own post) and a revenue grab (UN stamps can be sold to collectors and the United States has the expense of processing and delivering the United Nations Postal Administration's (UNPA) mail). Interest in UN stamps has always been a factor of interest in and respect for the United Nations as an organization, and it is safe to say that interest in UN stamps is at an all time low.
 
The first UN issues were issued in 1948 and were avidly collected, largely by Americans. For the first twenty years of UN issues, many US collectors maintained a collection of UN. It was cheap, and they could complete it. After the UN began taking an anti-US and anti-Israel line, beginning about 1970, UN philately increasingly fell out of favor. Three things have contributed to UN's continued decline in collector interest. First, collectors have been put off by the fact that now three UN postal agencies—New York, Geneva, and Vienna—all  issue stamps, making far too many issues for the legitimate postal needs of the UN. Second, the UNPA has continued to increase restrictions on the use of their stamps on mail (thus lowering the value of older postage type material). Originally, postal patrons could mail what they wanted from the UN. Now the post office only accepts smaller quantities of mail with difficult restrictions as to use. This has led to a steep decline in the value of older UN stamps to where it is hard to sell UN at 20% of the postage value. Collectors don't want to put their money into a collection that depreciates like this. And third, the investment scandal company Afinsa was gearing up to begin promoting UN before it was closed down by the Spanish government, leaving a market overhang of millions of dollars of UN stamps further depressing prices. It seems unlikely that the stamps of the United Nations will ever regain the popularity they once had.