Advertizing online for rentals available to tenants

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Does anyone know precisely how the various online websites interact? 

It appears that a "postlet" will upload to Zillow, Yahoo, hotpads and Trulia....according to how I read their process online.

Does Craiglist require the extra step to get on their site?  

What is getting the most traffic these days?

Seems like Craigslist is not as popular as previously?

Just started using these other methods and trying to weigh out the benefits.

Is there a simple way to get all the exposure?

Appreciate your insight!