I've have a few massive traffic surges from Reddit in the past, as much as a few 100k visitors, and have been wondering the same. Usually it amounts to just a handful of opt ins, fewer than I get from about 1k of organic traffic.

My impression is that people are most just interested in taking a peek before returning to reddit, so unlikely there is much you can do. I think the real value from scoring well on these sites comes not from the traffic, but from journalists and bloggers who monitor reddit for things to write about. I've found a high scoring link on reddit often leads to several secondary backlinks, which bring more engaged visitors.