From IndieWeb

This definition and page is both unclear in and of itself, and how it relates to the IndieWeb. If you understand "seenthis", please help fix this page! - Tantek 19:06, 21 February 2015 (PST)

Seenthis is a common (reference) that host content and help people curating it by using hashtags.

From there website:

"Short-blogging with no text limit. Curation and web survey. Cool automatic text layout. Forums under each post. Advanced and automatic thematisation."

It is interesting because you can comment on links, and by adding hashtags, it creates automatically a forum, or group, on this topic.

There is a nice article that describes deeper how it works by Thibault Henneton: fr Au fait, c’est quoi SeenThis ?