The crux of the problem is that, in most cases, social software is an extremely inefficient way for a person to get something done.People are lazy, appropriately so. If you ask them to do work, most of them won't do it. From their point of view, you're only of value to them if you save them time.
The crowd may enjoy the product of other people's inputs, but for the rather small group of individuals actually doing the work, it demands the investment of a lot of time for very little personal gain. It's a fun diversion for a while - and then it turns into drudgery.
It's very easy to confuse fads for trends ... Out in the real world, hardly anyone has even heard of Flickr or Digg or Delicious.
See also my previous posts, "Yahoo gets social with MyWeb", "Implicit vs. explicit sharing", and "Summing collective ignorance".