The majority of social media sites (with the exception of youtube imo) don't provide anything of value. A smart high school kid could write a twitter clone over a weekend. The only thing these websites have to offer is their large user base.
EDIT: OK, I apparently upset some people. I was exagerating when I said it could be done in a weekend, but my point stands that it's pretty easy to make a twitter clone/reddit clone, and the challenge in succeeding for twitter is getting a user base. The tech is incredibly easy to build.
lemonh3ad
themadcodger
Peacemeal12