Twitter, Facebook, G+, blog comments, web forums, &c. None of them seem able to handle one very important thing that NNTP clients did since the 1980s: Keep track of what I’ve read & what I haven’t.
The Twitter iOS client does a decent job of remembering my position in its time stream. That position doesn’t sync between my iPhone, my iPad, & the web site, though. Twitter is also not a great medium for conversation in many other ways, though.
Some web forums make an attempt at it, but I haven’t seen one that does it well. Although it has been a while since I was keeping up with forums.