Its true, facebook is making a new type of internet. Its hardly about search anymore, now people search facebook for content. Social networks are a fad like anything else but they are winning now.
The new layer is data locked into private site that farm content.
you do have to consider many of those searches as looking for stuff already in their social graph (friends, pages, etc). i know i use their search to navigate all the time.
Obviously at this point it is largely just offsetting personal email, but I don't think it's reasonable to dismiss that as insignificant.