I think you are right. They might be using a plethora of other factor as an addition to back links; however, genuine links from authoritative sources still seem to be the best "voting" mechanism to bubble up good content/results.

I can imagine that Reddit could join the search war - first, they have so much user gen content that people are deliberately looking for; second, they could use the voting mechanisms that are already in place to give preference to the "better" content. Of course, I might be wrong :).