I learned about Zillow late last year after my property had been vacant for four months and was ticked that I didn't know about it sooner. As soon as I posted to Zillow, the inquiries poured in! A Zillow Postlets listing also populates to affiliate services such as Hotpads and Trulia. But as a previous responder said, none of them post to the MLS.
TIP: This time around, I have been sending my rental criteria to all of the Zillow, Trulia, Postlets, Hotpads responders before I even give them my phone number. That has cut way down on the number of serious, qualified inquiries I need to respond to and schedule appointments with.
YMMV, Paula