I have this vague memory that investors or startup advisors used to laugh at any startup idea along the lines of "We want to build a social network, but for florists!"--and that seems to have receded in the last year.
The difference is that the new niche social networks are centered around selling stuff rather than making money from ads. Niche networks can't get big enough to make money from ads. They can get plenty big enough to sell stuff though.
I understood everything else you said, but I'm unsure what you mean by this. Could you explain?