One of the biggest problems that confronts anyone who wants to understand the meaning of UGC, especially for the purpose of targeted online ads, is Synonymy.  There are many ways to express the same idea and you can always count on the millions of social network users to use them all!  The incredible array of variant forms of words that all express the same underlying idea does make language very rich, but it creates further challenges for machine analysis of the interests and opinions of social network users.  For example, "The Stones", "The Rolling Stones"

One of the main challenges for those who are working with User Generated Content on the social web is its incredible malleability, variety and idiosyncratic nature.  In other words, UGC is a muddled up mess that rivals the Tower of Babel.  You all know what I am talking about.  Just look at a random MySpace profile and try to figure out what exactly is being expressed there.