Where do we go to discuss these?

I assume that was wondering if there are any websites out there which

• systematically provide space for the discussion of mathematics articles (particularly those on the arXiv, though not necessarily just those), and
• have a large enough user base to have some hope of having a particular paper good discussion (or at least have hope of attracting one in the near future).

Of course, MO is not appropriate for MOso far from such a site, but is organized around questions, not papers, which leads to rather different discussion in practice. Similarly, some blog posts and nLab pages serve this purpose, but are not created systematically.

Where do we go to discuss these? I assume that discussion of a particular paper is not appropriate for MO.