Seth points out another one

Regardless of whether this is a mesh-up of this and that site, or the exact twist that makes it different, or what this site does in particular, the interesting thing here are the powerful ideas behind it and how one can (apparently) re-use them, as long as the twist or the blend makes the new site different enough from the previous one in order to create that necessary critical mass (interest and initial traffic) to get the existing social networks diffuse the virus thorugh their favorite communication channels (oh... and yes, BTW, blogs are a very effective one, that's why there is always a blog in these sites).

As Seth points it, these sites will keep changing the rules. They are simple, cheap and extremelly fast and easy to build. They are genious in their very own way. There are just soooo many possible different twists and mesh-ups...