There is more US IPO activity going on right now than makes the headlines. You really have to subscribe to an IPO specific news source or blog.

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.)