[meritt]

Zillow and Trulia make the majority of their revenue selling preferential placement of realtor profiles and selling consumer leads to these realtors.

For Zillow 2014 Q1 it was $46.2M realtor revenue, $7.1M mortgage leads and $12.9M display advertising. Only about 20% of their revenue comes from ads.

[j10t]

I would label any form of "selling leads" as advertising.