I wish I'd known about the topic challenge Icelandic Sagas when it happened. Likewise, I'm sure there are people that are interested in some of the current challenges. Is there any way these challenges could be advertised across the SE network?
Yeah! You could post messages in relevant chat rooms, for example. Or if you know someone who might be interested, give them a ping. I would suggest community promotion ads, but we've already posted them.
Now that I think about it, chat events are annoying features that have a decent amount of visibility. Maybe we could try using those.
That said, our recruitment/promotion efforts are mostly focused on Twitter and Tumblr at the moment, and in my opinion rightfully so. Twitter and Tumblr have large communities of book/lit lovers, and these communities have hashtags/etc that allow us to reach a lot of people at once. The situation at SE is the opposite: smaller number of people who would be interested in our content, they're scattered across different sites, and the HNQ already does a better job getting our name out there than we ever could. (Of course, the HNQ isn't without its consequences, but that's a story for another day).