Take this with a grain of salt, I haven't read the S-1 and I don't know much about Zulily.
However, they have shown up as a top US display advertisers going back a year or more. Groupon was in the same spot, buying up massive quantities of inventory to fuel their growth. Before buying in take a close look at how much of their growth is purchased, along with the retention of those purchased users.
Other recent IPOs, like Retailmenot, have even deeper issues (almost complete dependency on free traffic from Google for trademark company names.)
Reddit didn't get a lot of press attention because -- among other things -- the people behind it were not easy to find and talk with. (They still aren't.) If as a journalist I can't connect with you to, say, fact check something I'm writing, your chances of being covered are much lower.
Plus, when Reddit was on the upswing, there were a _lot_ of competing sites that were trying to be "Digg for [niche]," such as DZone as "digg for developers" and dotnetkicks as "digg for the .net people so we can be even more granular," and agileblend and so on. Not to mention older sharing sites like delicious (which is, um, still around?). So initially it seemed like "yet another link sharing site," and one that was far more confusing even to us tech press geeks.
"Accessibility" has a lot to do with one's connections with journalists and perhaps location to some extent. It seems like a broken model is being used to determine what is newsworthy in tech.