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.
Obviously at this point it is largely just offsetting personal email, but I don't think it's reasonable to dismiss that as insignificant.
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.
The new layer is data locked into private site that farm content.